Fix possiblilty of branch not found

This now can happen because can_be_merged? is called
during MR merge_status transition to cannot_be_merged.
It is possible branch_name is invalid.

lib/gitlab/git/repository.rb

@@ -1413,8 +1413,11 @@ module Gitlab
       end
 
       def can_be_merged?(source_sha, target_branch)
-        target_sha = find_branch(target_branch, true).target
+        if target_sha = find_branch(target_branch, true)&.target
           !gitaly_conflicts_client(source_sha, target_sha).conflicts?
+        else
+          false
+        end
       end
 
       def search_files_by_name(query, ref)