Get the coherent Null SNR for this row.

def get_null_snr(self):
		"""
		Get the coherent Null SNR for this row.
		"""
		null_snr_sq = (numpy.asarray(self.get_sngl_snrs().values())**2)\
                             .sum() - self.snr**2
		if null_snr_sq < 0:
			return 0
		else:
			return null_snr_sq**(1./2.)