[mlir] Check for type conversion success in std->llvm function conversion

Type converter may fail and return nullptr on unconvertible types. The function
conversion did not include a check and was attempting to use a nullptr type to
construct an LLVM function, leading to a crash. Add a check and return early.
The rest of the call stack propagates errors properly.

Fixes PR47403.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D87075