Fix `nomask` handling and clarify docs.

This fixes a bug in `iris.experimental.regrid._regrid_bilinear_array()` to allow it to cope with degenerate masks. Previously it was checking `mask is not False` which fails to identify the degenerate case because in that case it's actually `numpy.bool_(0)` (see `np.ma.nomask`).

It also fixes the documentation to remove the false assertion that NaN values in the source data get converted to masked values. This change is backed by a new range of tests which explicitly check this behaviour.