Audit whether slicesext.ToStructMapOmitEmpty can be removed

[bufdev]

We have a note on slicesext.ToStructMapOmitEmpty:

buf/private/pkg/slicesext/slicesext.go

Line 149 in 705a37b

// TODO FUTURE: Make ToStructMap use this logic, remove ToStructMapOmitEmpty, to match other functions.

We likely did this to make sure we didn't edit existing behavior, but I bet if we go through buf and core, we won't find anywhere that relies on the zero value being populated as a key. We just need to do this audit, and if so, delete the existing ToStructMap and rename ToStructMapOmitEmpty to ToStructMap.