Allow pulp-only repos when requesting ODCS composes (OSBS-5462)

In the case where no packages are requested, no modules are requested,
but pulp_repos is true *and* there are Pulp content sets defined, do the
right thing.

Do not reject the build. Do not request package or module composes.
Request the Pulp composes.

Signed-off-by: Tim Waugh <twaugh@redhat.com>

atomic_reactor/plugins/pre_resolve_composes.py

@@ -346,7 +346,7 @@ def render_requests(self):
         requests = []
         if self.packages:
             requests.append(self.render_packages_request())
-        else:
+        elif self.modules:
             requests.append(self.render_modules_request())
 
         for arch in self.pulp:
@@ -382,8 +382,8 @@ def render_pulp_request(self, arch):
 
     def validate_for_request(self):
         """Verify enough information is available for requesting compose."""
-        if not self.packages and not self.modules:
-            raise ValueError('List of packages or modules cannot be empty')
+        if not self.packages and not self.modules and not self.pulp:
+            raise ValueError("Nothing to compose (no packages, modules, or enabled pulp repos)")
 
         if self.packages and self.modules:
             raise ValueError('Compose config cannot contain both packages and modules')

tests/plugins/test_resolve_composes.py

@@ -631,12 +631,17 @@ def test_signing_intent_multiple_composes(self, workflow, composes_intent, expec
         (dedent("""\
             compose:
                 packages: []
-            """), 'cannot be empty'),
+            """), 'Nothing to compose'),
 
         (dedent("""\
             compose:
                 modules: []
-            """), 'cannot be empty'),
+            """), 'Nothing to compose'),
+
+        (dedent("""\
+            compose:
+                pulp_repos: true
+            """), 'Nothing to compose'),
 
         (dedent("""\
             compose: