Null safety fails in some cases with safe navigation operator

**Reproduction:** https://try.haxe.org/#e7106E62

**Expected behavior:** Within the block of `if (test.first?.second?.third != null)`, `test.first.second.third` should be considered valid by the null safety checker.

**Actual behavior:** Within the block, the null check is not being properly considered.

Run the linked try.haxe.org reproduction first, then trace the compiler's null-safety handling for chained safe-navigation checks. Done means the condition in the example allows test.first.second.third inside the block without a null-safety error, while preserving the expected behavior of the reproduction.

Written by the indexing model from the issue text.