ci/apkbuild-linting: quit if there are no apkbuilds to lint (MR 2630)

If the only changed apkbuilds are skipped, then the check would lint
*every* apkbuild in aports (calling '-q lint' with no packages
apparently causes it to do everything)

This quits if all packages were skipped. It fixes the CI failure seen on
MR !2600

.ci/apkbuild-linting.py

@@ -19,6 +19,9 @@ if __name__ == "__main__":
             print(f"NOTE: Skipping linting of {apkbuild}")
             continue
         packages.append(os.path.basename(os.path.dirname(apkbuild)))
+    if len(packages) < 1:
+        print("No APKBUILDs to lint")
+        sys.exit(0)
 
     result = common.run_pmbootstrap(["-q", "lint"] + packages, output_return=True)