PhaseDiscriminator: handle special 0+0j case here

[jj1bdx]

This is a minimal modification to avoid inputting 0+0j to volk_32fc_s32f_atan2_32f().

Note: I'd rather want to remove this sort of hack altogether if my fix for VOLK 3.1.0 is accepted in the next VOLK release. After that, a possible workaround should be setting the corresponding volk_config entry as: volk_32fc_s32f_atan2_32f polynomial polynomial

[jj1bdx]

I think we don't need to treat 0+0j at the input of atan2 functions specifically.