fix: fix static resource retrieval with Jetty 12 (#18471) (#18508)

With Jetty 12, static resources retrieved via servlet context's `getResource(String)` may return non-null URL for non-existing resource. Jetty 11 returns null for non-existing resource.
This change adds a double check for the static resource retrieved with the getResource, and returns null if it doesn't exist, to keep other parts work the same way as with Jetty 11.

Fixes: #18410

Co-authored-by: Tomi Virtanen <tltv@vaadin.com>

flow-server/src/main/java/com/vaadin/flow/server/PwaRegistry.java

@@ -29,9 +29,12 @@
 import java.io.Serializable;
 import java.io.UncheckedIOException;
 import java.net.MalformedURLException;
+import java.net.URISyntaxException;
 import java.net.URL;
 import java.net.URLConnection;
 import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.LinkedHashSet;
@@ -161,7 +164,7 @@ private static Logger getLogger() {
 
     private URL getResourceUrl(ServletContext context, String path)
             throws MalformedURLException {
-        URL resourceUrl = context.getResource(path);
+        URL resourceUrl = VaadinServletService.getStaticResource(context, path);
         if (resourceUrl == null) {
             // this is a workaround specific for Spring default static resources
             // location: see #8705

flow-server/src/main/java/com/vaadin/flow/server/VaadinServletService.java

@@ -22,7 +22,10 @@
 
 import java.io.InputStream;
 import java.net.MalformedURLException;
+import java.net.URISyntaxException;
 import java.net.URL;
+import java.nio.file.Files;
+import java.nio.file.Path;
 import java.util.List;
 import java.util.Objects;
 import java.util.Optional;
@@ -240,7 +243,7 @@ public String resolveResource(String url) {
     @Override
     public URL getStaticResource(String path) {
         try {
-            return getServlet().getServletContext().getResource(path);
+            return getStaticResource(getServlet().getServletContext(), path);
         } catch (MalformedURLException e) {
             getLogger().warn("Error finding resource for '{}'", path, e);
         }
@@ -316,4 +319,22 @@ protected VaadinContext constructVaadinContext() {
     protected void setDefaultClassLoader() {
         setClassLoader(getServlet().getServletContext().getClassLoader());
     }
+
+    static URL getStaticResource(ServletContext servletContext, String path)
+            throws MalformedURLException {
+        URL url = servletContext.getResource(path);
+        if (url != null && Optional.ofNullable(servletContext.getServerInfo())
+                .orElse("").contains("jetty/12.")) {
+            // Making sure that resource exists before returning it. Jetty
+            // 12 may return URL for non-existing resource.
+            try {
+                if (!Files.exists(Path.of(url.toURI()))) {
+                    url = null;
+                }
+            } catch (URISyntaxException e) {
+                url = null;
+            }
+        }
+        return url;
+    }
 }