Fix namespace statistics migration failure

What does this MR do and why?

!79657 (merged) 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.

Relates to #352460 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.