Merge branch 'sh-fix-diff-check-issue-55137-ce' into 'master'

[CE] Fix DiffCheck failing due to invalid string argument

See merge request gitlab-org/gitlab-ce!23741

lib/gitlab/checks/diff_check.rb

@@ -11,6 +11,7 @@ module Gitlab
       }.freeze
 
       def validate!
+        return if deletion? || newrev.nil?
         return unless should_run_diff_validations?
         return if commits.empty?
         return unless uses_raw_delta_validations?
@@ -28,7 +29,7 @@ module Gitlab
       private
 
       def should_run_diff_validations?
-        newrev && oldrev && !deletion? && validate_lfs_file_locks?
+        validate_lfs_file_locks?
       end
 
       def validate_lfs_file_locks?