fix: Set default filename, when output file is empty

issue_metrics.py

@@ -171,15 +171,16 @@ def evaluate_markdown_file_size(output_file: str) -> None:
     """
     Evaluate the size of the markdown file and split it, if it is too large.
     """
-    file_name_without_extension = Path(output_file).stem
+    output_file_name = output_file if output_file else "issue_metrics.md"
+    file_name_without_extension = Path(output_file_name).stem
     max_char_count = 65535
-    if markdown_too_large_for_issue_body(output_file, max_char_count):
-        split_markdown_file(output_file, max_char_count)
-        shutil.move(output_file, f"{file_name_without_extension}_full.md")
-        shutil.move(f"{file_name_without_extension}_0.md", output_file)
+    if markdown_too_large_for_issue_body(output_file_name, max_char_count):
+        split_markdown_file(output_file_name, max_char_count)
+        shutil.move(output_file_name, f"{file_name_without_extension}_full.md")
+        shutil.move(f"{file_name_without_extension}_0.md", output_file_name)
         print(
             f"Issue metrics markdown file is too large for GitHub issue body and has been \
-split into multiple files. ie. {output_file}, {file_name_without_extension}_1.md, etc. \
+split into multiple files. ie. {output_file_name}, {file_name_without_extension}_1.md, etc. \
 The full file is saved as {file_name_without_extension}_full.md\n\
 See https://github.com/github/issue-metrics/blob/main/docs/dealing-with-large-issue-metrics.md"
         )

test_issue_metrics.py

@@ -482,6 +482,18 @@ def test_get_per_issue_metrics_with_discussion_with_hide_envs(self):
 class TestEvaluateMarkdownFileSize(unittest.TestCase):
     """Test suite for the evaluate_markdown_file_size function."""
 
+    @patch("issue_metrics.markdown_too_large_for_issue_body")
+    def test_markdown_too_large_for_issue_body_called_with_empty_output_file(
+        self, mock_evaluate
+    ):
+        """
+        Test that the function uses the output_file.
+        """
+        mock_evaluate.return_value = False
+        evaluate_markdown_file_size("")
+
+        mock_evaluate.assert_called_with("issue_metrics.md", 65535)
+
     @patch("issue_metrics.markdown_too_large_for_issue_body")
     def test_markdown_too_large_for_issue_body_called_with_output_file(
         self, mock_evaluate