Account for unset attribute

If the repofile isn't set at all, this will fail

Author: kellyma2

bazeldnf/extensions.bzl

@@ -107,7 +107,7 @@ def _alias_repository_impl(repository_ctx):
     if repository_ctx.attr.lock_file.package:
         lock_file_path = repository_ctx.attr.lock_file.package + "/" + lock_file_path
 
-    if repository_ctx.attr.repofile.package:
+    if repository_ctx.attr.repofile and repository_ctx.attr.repofile.package:
         repofile = repository_ctx.attr.repofile.package + "/" + repofile
 
     repository_ctx.file(