fix: add error description to batch emptiness validation (#2109)

When invoking BatchRequest#execute(), we ensure the batch contains at
least one request to process. Previously this check, did not provide any
error message. Now, if the check fails "Batch is empty" will be the
message of the resulting exception.

Related to googleapis/java-storage#694

Author: BenWhitehead

google-api-client/src/main/java/com/google/api/client/googleapis/batch/BatchRequest.java

@@ -228,7 +228,7 @@ public int size() {
    */
   public void execute() throws IOException {
     boolean retryAllowed;
-    Preconditions.checkState(!requestInfos.isEmpty());
+    Preconditions.checkState(!requestInfos.isEmpty(), "Batch is empty");
 
     // Log a warning if the user is using the global batch endpoint. In the future, we can turn this
     // into a preconditions check.