Remove unnecessary handling of nil paths in HiddenAPIOutput

Previously, bootclasspath_fragment modules could produce
HiddenAPIOutput structs containing nil paths. That is no longer
possible as both source and prebuilt bootclasspath_fragment modules
always provide non-nil paths so this change removes the special
handling.

Bug: 194063708
Test: m out/soong/hiddenapi/hiddenapi-flags.csv
      - check that this does not change the contents.
Change-Id: I15337e146c9e694ce9da06ad4845ac267d6da27a