chore: do not create pr description without qualified commit (#2806)

In this PR:
- Do not generate `pr_description.txt` if no qualified commit found
between two googleapis commit.

library_generation/generate_pr_description.py

@@ -22,6 +22,8 @@
 from library_generation.utils.commit_message_formatter import format_commit_message
 from library_generation.utils.commit_message_formatter import wrap_override_commit
 
+EMPTY_MESSAGE = ""
+
 
 def generate_pr_descriptions(
     config: GenerationConfig,
@@ -62,6 +64,10 @@ def generate_pr_descriptions(
         is_monorepo=config.is_monorepo(),
     )
 
+    if description == EMPTY_MESSAGE:
+        print("Empty commit messages, skip creating pull request description.")
+        return
+
     description_file = f"{description_path}/pr_description.txt"
     print(f"Writing pull request description to {description_file}")
     with open(description_file, "w+") as f:
@@ -115,6 +121,9 @@ def get_commit_messages(
             break
         commit = commit_parents[0]
     shutil.rmtree(tmp_dir, ignore_errors=True)
+    if len(qualified_commits) == 0:
+        return EMPTY_MESSAGE
+
     return __combine_commit_messages(
         current_commit=current_commit,
         baseline_commit=baseline_commit,

library_generation/test/generate_pr_description_unit_tests.py

@@ -70,3 +70,28 @@ def test_generate_pr_description_with_same_googleapis_commits(self):
             description_path=cwd,
         )
         self.assertFalse(os.path.isfile(f"{cwd}/pr_description.txt"))
+
+    def test_generate_pr_description_does_not_create_pr_description_without_qualified_commit(
+        self,
+    ):
+        # committed on May 22nd, 2024
+        old_commit_sha = "30717c0b0c9966906880703208a4c820411565c4"
+        # committed on May 23rd, 2024
+        new_commit_sha = "eeed69d446a90eb4a4a2d1762c49d637075390c1"
+        cwd = os.getcwd()
+        generate_pr_descriptions(
+            config=GenerationConfig(
+                gapic_generator_version="",
+                googleapis_commitish=new_commit_sha,
+                libraries_bom_version="",
+                template_excludes=[],
+                grpc_version="",
+                protoc_version="",
+                # use empty libraries to make sure no qualified commit between
+                # two commit sha.
+                libraries=[],
+            ),
+            baseline_commit=old_commit_sha,
+            description_path=cwd,
+        )
+        self.assertFalse(os.path.isfile(f"{cwd}/pr_description.txt"))