Allow efac/log10_tneqad to be None in ndiag functions

[mattpitkin]

In white_signals.py, the *_ndiag functions fail if they get passed None values. This happens, for example, if the white_noise_block in
enterprise_extensions has vary=False, and therefore efac/equad/ecorr are set to Constant parameters, which have default values of None.

The two potential fixes are the one in this PR, i.e., allow None values, which for efac_ndiag is equivalent to having efac=1.0, and set tneqad_ndiag to zeros if passed None. The other fix would be to specify values for the Constantsin thewhite_noise_block` in
enterprise_extensions, e.g.,:

        efac = parameter.Constant(1.0)
        equad = parameter.Constant(-np.inf)
        if inc_ecorr:
            ecorr = parameter.Constant(-np.inf)

[vallis]

These functions expect numerical values, and they have even defaults for them. I am not sure it's appropriate to give them flow branches to handle None. (Doing so, among other things, may prevent code compilation in some frameworks.) I would prefer a fix in e_e.

[mattpitkin]

@vallis No problem. I'll close this and submit a PR for enterprise_extensions.