None gets dropped in ternary statement with lists

[ddfisher]

python3 -m mypy --strict-optional -c 'reveal_type([None] if bool() else [1])' <string>:1: error: Revealed type is 'builtins.list[builtins.int]'

[refi64]

This is basically just a duplicate of #2246. is_valid_inferred_type is False for [None] since it's technically invalid without a type annotation, so visit_conditional_expr just discards the type of that branch. Pretty sure fixing #2246 would also fix this.

[ddfisher]

Nice catch! It looks that way to me too. This makes me feel like our decision on #2246 is definitely right.