Absence of representation comparison makes frame.is_equivalent_frame incorrectly fail #7793

In [1]: from astropy.coordinates import CartesianRepresentation

In [2]: CartesianRepresentation(x=0, y=0, z=0, unit="km") == CartesianRepresentation(x=0, y=0, z=0, unit="km")
Out[2]: False

In [3]: CartesianRepresentation(x=0, y=0, z=0, unit="km") == CartesianRepresentation(x=1000, y=1000, z=1000, unit="km")
Out[3]: False

In [4]: from astropy.coordinates import GCRS, HCRS

In [5]: from astropy.time import Time

In [6]: epoch = Time.now()

In [7]: HCRS(obstime=epoch).is_equivalent_frame(HCRS(obstime=epoch))
Out[7]: True

In [8]: GCRS(obstime=epoch).is_equivalent_frame(GCRS(obstime=epoch))
Out[8]: False

In [9]: GCRS(obstime=epoch).obsgeoloc
Out[9]: 
<CartesianRepresentation (x, y, z) in m
    (0., 0., 0.)>

In [10]: GCRS(obstime=epoch).obsgeoloc == GCRS(obstime=epoch).obsgeoloc
Out[10]: False