[blame] Don't pass null to PersonIdent constructor

The API was changed to not allow null values anymore with
I0ac994ae8e47789d38f7c6e6db55d482f0f1bac3, leading to an IAE.

Bug: 393054
Change-Id: If33560ae976b46a02ff75b2e4ec05c13a8ad2d41

org.eclipse.jgit/src/org/eclipse/jgit/blame/Candidate.java

@@ -380,7 +380,7 @@ class Candidate {
 
 		@Override
 		PersonIdent getAuthor() {
-			return new PersonIdent(description, null);
+			return new PersonIdent(description, "");
 		}
 	}
 }