Check if soong_config_var propertystruct is zero before panicking

If we have a soong_config_module_type with target.android_<arch>.*
properties, we should only panic if a soong_config_var is used to select
on those properties. We already call `IsZero` in an upper function, but
we need to call it in `AddSoongConfigPropertiesFromTargetStruct` as well
because conditions_default gets its own struct.

Test: unit tests
Test: repro'd cmd in b/293206386#comment1, it passes now
Bug: 293206386
Change-Id: I23f14f83cc9be66487625e292e26b204dfea4f9a