fix: don't open bug when .OwlBot.yaml is missing (googleapis#5244)

The contents of the repo are the source of truth.
scan-googleapis-gen-and-create-pull requests queries firestore to find
.OwlBot.yaml files. Firestore may have stale data. So if Owl Bot queries
firestore, finds an .OwlBot.yaml, and then can't find the .OwlBot.yaml
in the repo, then that's not actually a problem.

Fixes googleapis#5242

---------

Co-authored-by: Jeff Ching <chingor@google.com>

packages/owl-bot/src/copy-code.ts

@@ -190,6 +190,10 @@ export async function copyCodeIntoCommit(
     const localYamlPath = path.join(params.destDir, yamlPath);
     let yaml: OwlBotYaml | undefined;
     try {
+      if (!fs.existsSync(localYamlPath)) {
+        logger.warn(`${localYamlPath} doesn't exist.`);
+        continue;
+      }
       yaml = await loadOwlBotYaml(localYamlPath);
     } catch (e) {
       await reportError(e, yamlPath);
@@ -547,7 +551,7 @@ export async function copyCodeAndAppendOrCreatePullRequest(
       const issue = await octokit.issues.create({
         owner: params.destRepo.owner,
         repo: params.destRepo.repo,
-        title: `${yamlPath} is missing or defective`,
+        title: `${yamlPath} is defective`,
         body: `While attempting to copy files from
 ${sourceLink}