This is follow-up from #41437 and it reduces the amount of code we generate for safe property accesses (`a?.b`) and nullish coalescing (`a ?? b`) by:
1. Reusing variables in nested nullish coalescing expressions.
2. Not initializing temporary variables to `null`. The way our code is generated means that the value will always be overwritten before we compare against it so the initializer didn't really matter.
Fixes#41491.
PR Close#41563