[MemProf] Handle empty stack context during ThinLTO cloning (#81008)

Fix for assert after PR#78264.

Handle the case where the MIB context is empty after skipping the
callsite context, because the callsite context is actually longer than
the MIB context. Presumably this happened as a result of inlining, but
in theory the metadata should have been replaced with an attribute in
that case. Need to investigate why this is occuring, but for now handle
this gracefully to fix the build regression.