In some cases non-null non-constant yet valid expression may reach point where
`ConditionResult` is created. For example, typo correction mechanism can return
such expression, so double check before evaluating it.
Fixes https://github.com/llvm/llvm-project/issues/61885
Reviewed By: tbaeder, aaron.ballman
Differential Revision: https://reviews.llvm.org/D148206
