scm: git: fix tests fail when extra_info is nil (#12409)

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@10857 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/models/repository/git.rb

@@ -257,6 +257,7 @@ class Repository::Git < Repository
   end
 
   def clear_extra_info_of_changesets
+    return if extra_info.nil?
     v = extra_info["extra_report_last_commit"]
     write_attribute(:extra_info, nil)
     h = {}