fix(serverless): Check if cloud event callback is a function (#9044)

Author: tcoyze

packages/google-cloud-serverless/src/gcpfunction/cloud_events.ts

@@ -56,7 +56,9 @@ function _wrapCloudEventFunction(
               DEBUG_BUILD && logger.error(e);
             })
             .then(() => {
-              callback(...args);
+              if (typeof callback === 'function') {
+                callback(...args);
+              }
             });
         });