build(python): exit with success status if no samples found (#1444)

Source-Link: googleapis/synthtool@53ea389
Post-Processor: gcr.io/repo-automation-bots/owlbot-python:latest@sha256:e1793a23ae0ee9aafb2e3a53b564a351f74790dbe3c2d75f8fc3b8c43e5c036c

Author: gcf-owl-bot[bot]

.github/.OwlBot.lock.yaml

@@ -1,3 +1,3 @@
 docker:
   image: gcr.io/repo-automation-bots/owlbot-python:latest
-  digest: sha256:df50e8d462f86d6bcb42f27ecad55bb12c404f1c65de9c6fe4c4d25120080bd6
+  digest: sha256:e1793a23ae0ee9aafb2e3a53b564a351f74790dbe3c2d75f8fc3b8c43e5c036c

.kokoro/test-samples-impl.sh

@@ -20,9 +20,9 @@ set -eo pipefail
 # Enables `**` to include files nested inside sub-folders
 shopt -s globstar
 
-# Exit early if samples directory doesn't exist
-if [ ! -d "./samples" ]; then
-  echo "No tests run. `./samples` not found"
+# Exit early if samples don't exist
+if ! find samples -name 'requirements.txt' | grep -q .; then
+  echo "No tests run. './samples/**/requirements.txt' not found"
   exit 0
 fi