build(python): samples tests should pass if no samples exist (#803)

synthtool/gcp/templates/python_library/.kokoro/test-samples.sh

@@ -28,6 +28,12 @@ if [[ $KOKORO_BUILD_ARTIFACTS_SUBDIR = *"periodic"* ]]; then
     git checkout $LATEST_RELEASE
 fi
 
+# Exit early if samples directory doesn't exist
+if [ ! -d "./samples" ]; then
+  echo "No tests run. `./samples` not found"
+  exit 0
+fi
+
 # Disable buffering, so that the logs stream through.
 export PYTHONUNBUFFERED=1
 
@@ -101,4 +107,4 @@ cd "$ROOT"
 # Workaround for Kokoro permissions issue: delete secrets
 rm testing/{test-env.sh,client-secrets.json,service-account.json}
 
-exit "$RTN"
+exit "$RTN"