checks: Introduce `#blank_rev?` helper

We have multiple locations which check for a blank revision, which
either is a blank value or the zero OID. We're about to add one more
location, so let's prepare for this and create a helper function to
deduplicate the logic.

lib/gitlab/checks/changes_access.rb

@@ -35,7 +35,7 @@ module Gitlab
       def commits
         newrevs = @changes.map do |change|
           newrev = change[:newrev]
-          newrev unless newrev.blank? || Gitlab::Git.blank_ref?(newrev)
+          newrev unless blank_rev?(newrev)
         end.compact
 
         return [] if newrevs.empty?
@@ -80,7 +80,7 @@ module Gitlab
         @single_changes_accesses ||=
           changes.map do |change|
             commits =
-              if change[:newrev].blank? || Gitlab::Git.blank_ref?(change[:newrev])
+              if blank_rev?(change[:newrev])
                 []
               else
                 Gitlab::Lazy.new { commits_for(change[:newrev]) }
@@ -109,6 +109,10 @@ module Gitlab
       def bulk_access_checks!
         Gitlab::Checks::LfsCheck.new(self).validate!
       end
+
+      def blank_rev?(rev)
+        rev.blank? || Gitlab::Git.blank_ref?(rev)
+      end
     end
   end
 end