[flang] Add explicit nullptr check in initialisation of inDoConstruct.

This explicit check is needed as we are using braced initialisation here so
implicit narrowing conversions (such as pointer to bool) are not allowed.

Original-commit: flang-compiler/f18@ff8dad10bffb70666ba3b4ff229dc4b608f9e116
Reviewed-on: https://github.com/flang-compiler/f18/pull/1083