Submit: Handle potentially missing username

Bug: Issue 11112
Change-Id: Ie5ed372b6cf890a9cebd5860af8905dedb306e6a

src/main/java/com/googlesource/gerrit/plugins/hooks/Submit.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/hooks/Submit.java b/src/main/java/com/googlesource/gerrit/plugins/hooks/Submit.java
index 7474553..e383200 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/hooks/Submit.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/hooks/Submit.java
@@ -51,7 +51,7 @@
     args.add("--project", projectName);
     args.add("--branch", destBranch.get());
     args.add("--submitter", caller.getNameEmail());
-    args.add("--submitter-username", caller.getUserName().get());
+    args.add("--submitter-username", caller.getUserName().orElse(null));
     args.add("--patchset", patchSetId.get());
     args.add("--commit", commit.getId().name());