return [] if error occured on branches or tags check

other parts of code expects array and test it with empty? method

Author: korin

Diff for: lib/redmine/scm/adapters/xitolite_adapter.rb

@@ -92,7 +92,7 @@ def branches
           end
           @branches.sort!
         rescue ScmCommandAborted
-          nil
+          []
         end
 
 
@@ -105,7 +105,7 @@ def tags
           end
           @tags
         rescue ScmCommandAborted
-          nil
+          []
         end