[11.x] fix: improve performance and robustness of Relation::getMorphAlias()

[calebdw]

Hello!

Please see #51809 (comment) for context.

Thanks!

[shengslogar]

Coming off of the parent comment thread, was there any discussion about calling this getMorphClass to match the name of HasRelationships->getMorphClass()? It would make more sense to me to call it getMorphAlias if we weren't falling back to the class name (like the original PR). In its current state, it seems like these methods should reference each other.