Edit: If a client doesn't fill out the LatLng message, that's different from lat and/or lon being null or 0. The whole LatLng message itself will be null. Proto3 always supported that too. But it's usually overkill to check the message being null, unless you added it later and need to specially deal with clients predating that change. If the client just has a bug preventing it from filling out LatLng, that's the client's problem.

The confusing part here is that even if the LatLng message is null, LatLng.lat will return 0 in some proto libraries, depending on the language. You have to specifically check if the message is null if you care. But there are enough cases where you have tons of nested protos and the 0-default behavior is actually way more convenient.