[flang] Fix optional assertion in PFTBuilder

D142279 enabled assertion in libstdc++ and one was triggered
in the PFTBuilder because an optional was access even if it was
null.
This patch fix this issue and add a regression test.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D143589

(cherry picked from commit 3f55311a)