Tentative fix for the init in HelioProjective class.
The presence and acceptability of FrameAttribute instances in the constructor complicate things.
Added an if clause inside the elif not args clause, which basically looks for if Tx and Ty have not been specified (the basis for any Spherical/UnitSpherical rep).
If not specified, it is the empty frame case. Else, proceed to add 'zeta' to kwargs.
The args case does not deal with this problem of HelioProjective(d=something, D0=something) because the first two args will always be Tx, Ty. d and D0 must be specified as keywords? Check this case.