Return false if there are no existent inputs to GenerateResource (#9208)

Fixes #9199 by avoiding an unhandled exception:

```
"C:\temp\testgenres\project.proj" (default target) (1) ->
  MSBUILD : error : This is an unhandled exception in MSBuild -- PLEASE UPVOTE AN EXISTING ISSUE OR FILE A NEW ONE AT https://aka.ms/msbuild/unhandled [C:\temp\testgenres\project.proj]
MSBUILD : error :     Microsoft.Build.Framework.InternalErrorException: MSB0001: Internal MSBuild Error: There should be resource files to process [C:\temp\testgenres\project.proj]
MSBUILD : error :    at Microsoft.Build.Shared.ErrorUtilities.ThrowInternalError(String message, Exception innerException, Object[] args) [C:\temp\testgenres\project.proj]
MSBUILD : error :    at Microsoft.Build.Tasks.GenerateResource.TransformResourceFilesUsingResGen(List`1 inputsToProcess, List`1 outputsToProcess) [C:\temp\testgenres\project.proj]
MSBUILD : error :    at Microsoft.Build.Tasks.GenerateResource.GenerateResourcesUsingResGen(List`1 inputsToProcess, List`1 outputsToProcess) [C:\temp\testgenres\project.proj]
MSBUILD : error :    at Microsoft.Build.Tasks.GenerateResource.Execute() [C:\temp\testgenres\project.proj]
```

src/Tasks.UnitTests/ResourceHandling/GenerateResource_Tests.cs

@@ -11,6 +11,7 @@
 using Microsoft.Build.Framework;
 using Microsoft.Build.Shared;
 using Microsoft.Build.Tasks;
+using Microsoft.Build.UnitTests.Shared;
 using Microsoft.Build.Utilities;
 using Shouldly;
 using Xunit;
@@ -338,7 +339,7 @@ public void ForceOutOfDate()
             /* Unmerged change from project 'Microsoft.Build.Tasks.UnitTests (net7.0)'
             Before:
             Utilities.AssertLogContainsResource(t, "GenerateResource.OutputDoesntExist", t.OutputResources[0].ItemSpec);
-            
+
             Utilities.AssertStateFileWasWritten(t);
             After:
             Utilities.AssertLogContainsResource(t, "GenerateResource.OutputDoesntExist", t.OutputResources[0].ItemSpec);
@@ -1718,7 +1719,7 @@ public void FailedResXReader()
                 /* Unmerged change from project 'Microsoft.Build.Tasks.UnitTests (net7.0)'
                 Before:
                 Assert.False(success);
-                
+
                 Utilities.AssertStateFileWasWritten(t);
                 After:
                 Assert.False(success);
@@ -1797,7 +1798,7 @@ public void FailedResXReaderWithAllOutputResourcesSpecified()
                 /* Unmerged change from project 'Microsoft.Build.Tasks.UnitTests (net7.0)'
                 Before:
                 Assert.False(success);
-                
+
                 Utilities.AssertStateFileWasWritten(t);
                 After:
                 Assert.False(success);
@@ -2317,7 +2318,7 @@ public void OutputFilesPartialInputs()
                 /* Unmerged change from project 'Microsoft.Build.Tasks.UnitTests (net7.0)'
                 Before:
                 Assert.Equal(t.FilesWritten[2].ItemSpec, Path.ChangeExtension(t.Sources[3].ItemSpec, ".resources"));
-                
+
                 Utilities.AssertStateFileWasWritten(t);
                 After:
                 Assert.Equal(t.FilesWritten[2].ItemSpec, Path.ChangeExtension(t.Sources[3].ItemSpec, ".resources"));
@@ -3653,6 +3654,20 @@ GenerateResource ExecuteTask()
                 Utilities.FileUpdated(resourcesFile, initialWriteTime).ShouldBeFalse();
             }
         }
+
+        /// <summary>
+        /// https://github.com/dotnet/msbuild/issues/9199
+        /// </summary>
+        [Fact]
+        public void NotValidSources()
+        {
+            GenerateResource t = new GenerateResource { BuildEngine = new MockEngine(_output) };
+            t.Sources = new ITaskItem[] { new TaskItem("non-existent") };
+            t.OutputResources = new ITaskItem[] { new TaskItem("out") };
+            Assert.False(t.Execute());
+            ((MockEngine)t.BuildEngine).AssertLogContains("MSB3552");
+            Assert.Equal(1, ((MockEngine)t.BuildEngine).Errors);
+        }
     }
 }
 

src/Tasks/GenerateResource.cs

@@ -714,14 +714,22 @@ public override bool Execute()
 
                 GetResourcesToProcess(out inputsToProcess, out outputsToProcess, out cachedOutputFiles);
 
-                if (inputsToProcess.Count == 0 && !Log.HasLoggedErrors)
+                if (inputsToProcess.Count == 0)
                 {
-                    if (cachedOutputFiles.Count > 0)
+                    if (!Log.HasLoggedErrors)
                     {
-                        OutputResources = cachedOutputFiles.ToArray();
-                    }
+                        if (cachedOutputFiles.Count > 0)
+                        {
+                            OutputResources = cachedOutputFiles.ToArray();
+                        }
 
-                    Log.LogMessageFromResources("GenerateResource.NothingOutOfDate");
+                        Log.LogMessageFromResources("GenerateResource.NothingOutOfDate");
+                    }
+                    else
+                    {
+                        // No valid sources found--failures should have been logged in GetResourcesToProcess
+                        return false;
+                    }
                 }
                 else if (FailIfNotIncremental)
                 {