bp2build correctly handle empty soong_config_var

Previously in cases where a variable value was set but empty in a bp
file we ignored the value in bp2build; however, the correct behavior is
to take the empty value for each case. The tests for soong config vars
with empty_prop_for_string_var and unused_string_var:
https://cs.android.com/android/platform/superproject/+/master:build/soong/android/soong_config_modules_test.go;l=243-283;drc=72beb34609217f3c98623fb6795b6417c7f0fc65
highlight the difference in when conditions default should be used or
not.

Test: go test bp2build tests
Change-Id: I9768241f013a1a1bc8fa23657a066b5b3f52f09b