Fix local var error, warn if no outputs, handle "}generate{"

1. UnboundLocalError: cannot access local variable 'output' where
   it is not associated with a value

2. `build/soong/tests/genrule_sandbox_test.py WmediumdServerProto_cc`
   would try to build the entire world due to 3)

3. WmediumdServerProto_cc 's module action desc had '}generate{'
   without a space which would cause 2)

Bug: b/287470077
Test: build/soong/tests/genrule_sandbox_test.py WmediumdServerProto_cc
Change-Id: Ie9d5db44195a6f700cc876ec889bd01c11e66cff