Allow empty string as max object size limit

An empty string for the max object size limit is now handled in
the same way as a null value.

Change-Id: Ib52ef79bdd6338a6b9f18821a510ccbf6816cc98
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

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

@@ -751,6 +751,10 @@ public class ProjectConfig extends VersionedMetaData {
     if (value == null) {
       return null;
     }
+    value = value.trim();
+    if (value.isEmpty()) {
+      return null;
+    }
     Config cfg = new Config();
     cfg.fromText("[s]\nn=" + value);
     try {