Initialize the submodule commit message buffer

Appending to a String that is null causes the null value to be coerced
to a string literal, such that the previously-null String value becomes
a string literal "null".

Change-Id: I07a4b071fabbb0b4a28403603884cb05d777c01b
Bug: Issue 1396

gerrit-server/src/main/java/com/google/gerrit/server/git/SubmoduleOp.java

@@ -202,6 +202,9 @@ public class SubmoduleOp {
       if (!subscribers.isEmpty()) {
         String msgbuf = msg;
         if (msgbuf == null) {
+          // Initialize the message buffer
+          msgbuf = "";
+
           // The first updatedBranch on a cascade event of automatic
           // updates of repos is added to updatedSubscribers set so
           // if we face a situation having