Fix a nill pointer dereference when TARGET_FLATTEN_APEX

When TARGET_FLATTEN_APEX is set to true, there is no single output file
for an APEX that other modules can reference via ":module" syntax.
Return nothing in that case.

Bug: 121221006
Test: TARGET_FLATTEN_APEX=true m
Change-Id: I556b55073720b16fd30fd133af58aac229e958c0