Allow label values to be configured with no text

Internal Server Error is raised during Gerrit startup if a project
is configured with a label that has a value with no text:

[label My-Label]
  value = -1 Failed
  value = 0
  value = +1 Passed

(Note that there is no text after "value = 0".)

Internal Server Error is raised due to IndexOutOfBoundsException
while constructing a LabelValue and assuming that the label has
both a numeric value and a text value.

Fix this by checking if there is a text value, and if there is no
text value, defaulting to an empty string.

Change-Id: Ib2f0532e64be745ba1ed83c372e804743a5afa7e

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

@@ -528,9 +528,10 @@ public class ProjectConfig extends VersionedMetaData {
     if (parts.isEmpty()) {
       throw new IllegalArgumentException("empty value");
     }
+    String valueText = parts.size() > 1 ? parts.get(1) : "";
     return new LabelValue(
         Shorts.checkedCast(PermissionRule.parseInt(parts.get(0))),
-        parts.get(1));
+        valueText);
   }
 
   private void loadLabelSections(Config rc) throws IOException {