query: Empty branch is the constant TRUE

0a9e7c0 evaluated both the empty branch and
empty repo as the constant true. However, empty repo was treated as an indicator
to display the list of all repos, so broke that functionality. However, there is
no such special casing for branch.

Change-Id: I15bc8617486211cecabcbf4d26a602b017f80fe0

query/query.go

@@ -281,6 +281,10 @@ func evalConstants(q Q) Q {
 		if s.Regexp.Op == syntax.OpEmptyMatch {
 			return &Const{true}
 		}
+	case *Branch:
+		if s.Pattern == "" {
+			return &Const{true}
+		}
 	}
 	return q
 }