[mlir] fix dereferencing of optional sym_name attribute

`sym_name` is an optional attribute of `ModuleOp`, so it is unsafe to
fetch the underlying value without checking whether it is non-empty.
Such unsafe dereferencing causes the lower-host-to-llvm-calls_fail.mlir
test to segfault.  Although this bug existed for four months, it wasn't
triggered, since previous tests executed a code path that used a default
value instead of one fetched from the module attribute.

This patch makes the code use a default value if the optional attribute
does not have a value.

Reviewed By: stella.stamenova

Differential Revision: https://reviews.llvm.org/D141941