find_static_candidates.py: protect against missing keys in module

Looks like modules can now be missing "shared_libs" and "static_libs".
Use default values of empty sets where applicable, or check for the
existence of the keys before looking for them.

Test: python find_static_candidates.py --module audioserver
Bug: 322190716
Change-Id: I8be9bd2c1562975db57ffbf89d57ff2767f0b1f1