[Size reports] Fix transient failure (#14545)

#### Problem

The workflow to produce size report comments on PRs adds new
data to existing comments as CI runs complete. This fails when
(a) there is an existing partial comment for a PR commit, and
(b) there are new build artifacts for the commit and/or master, but
(c) none of the new artifact pairs are for the same specific builds.
In this case there is no new data to add, which leads to an internal
table column having the wrong dynamic data type, which causes a
python exception.

#### Change overview

Don't try to create or amend a GitHub comment when there is no new data.

#### Testing

Locally exercised using current build artifacts.

scripts/tools/memory/gh_report.py

@@ -691,6 +691,9 @@ def report_matching_commits(db: SizeDatabase) -> Dict[str, pd.DataFrame]:
             continue
 
         df = changes_for_commit(db, pr, commit, parent)
+        if df.empty:
+            # Matching commits had no new matching builds.
+            continue
         dfs[df.attrs['name']] = df
 
         if (event == 'pull_request' and comment_enabled