Don't panic in turbine with no classes

ALLOW_MISSING_DEPENDENCIES=true can cause missing dependencies to be
hidden from GenerateAndroidBuildActions, which is triggering the
len(jars) == 0 check in compileJavaHeader.  For the
ALLOW_MISSING_DEPENDENCIES=true case the check is unnecessary, as the
build rules will be replaced with runtime errors.  On the off chance
the check would have been hit in a real build, allow merge_zips to
create an empty zip file if it is asked to.

Test: m ALLOW_MISSING_DEPENDENCIES=true
Change-Id: I3171f921d51229ddf38a1a647d32566658c673fa