Fix namespace statistics migration failure

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/79657 modified
`NamespaceStatistics` to use a new database column
(`dependency_proxy_size`) in the `before_save` callback. However,
migration tests that roll back to an older schema value previously
would fail since the column did not exist.

To mitigate this issue, we now check for the existence of the column
before attempting to use it.

app/models/namespace_statistics.rb

@@ -29,6 +29,10 @@ class NamespaceStatistics < ApplicationRecord # rubocop:disable Gitlab/Namespace
   end
 
   def update_storage_size
+    # This prevents failures with older database schemas, such as those
+    # in migration specs.
+    return unless self.class.database.cached_column_exists?(:dependency_proxy_size)
+
     self.storage_size = dependency_proxy_size
   end