[AsmPrinter] Make OpAsmPrinter::printFunctionalType be resilient to null values.

A previous patch made Value::getType() be resilient to null values which was
considered to be too sweeping.  This is a more targeted change which requires
deabstracting some templates.

A middle ground would be to make ValueTypeIterator be tolerant to null values.

Differential Revision: https://reviews.llvm.org/D93908