Allow generate_hiddenapi_lists.py to work with no custom flag files

Previously, if generate_hiddenapi_lists.py was invoked without any
custom flag files, e.g. hiddenapi-unsupported.txt, then it would fail.
This change allows it to be used for generating the hiddenapi lists
for modules that do not have any custom flag files,
e.g. framework-sdkextensions

Bug: 179354495
Test: verified that the monolithic out/soong/hiddenapi/... files are
      unchanged by this change
Change-Id: I455d453024c9f06ed59cbc1e9838234f8b7c7317