Invalid TRPOS<n> keyword generated for FITS time column #8773
Description
It appears that in simple cases the Time FITS machinery is generating a blank value for the TRPOS<n> keyword that specifies location. I think for no location this keyword should be removed. This came up in https://github.com/astropy/astropy/pull/8754/files#r287430548.
In [14]: tm = Time(['2001-01-01'])
In [15]: t = Table([tm])
In [16]: t.write('time.fits', overwrite=True)
In [17]: !fitsheader time.fits
# HDU 0 in time.fits:
SIMPLE = T / conforms to FITS standard
BITPIX = 8 / array data type
NAXIS = 0 / number of array dimensions
EXTEND = T
# HDU 1 in time.fits:
XTENSION= 'BINTABLE' / binary table extension
BITPIX = 8 / array data type
NAXIS = 2 / number of array dimensions
NAXIS1 = 16 / length of dimension 1
NAXIS2 = 1 / length of dimension 2
PCOUNT = 0 / number of group parameters
GCOUNT = 1 / number of groups
TFIELDS = 1 / number of table fields
TIMESYS = 'UTC ' / Default time scale
JDREF = 0.0 / Time columns are jd = jd1 + jd2
TREFPOS = 'TOPOCENTER' / Time reference position
TTYPE1 = 'col0 '
TFORM1 = '2D '
TUNIT1 = 'd '
TCTYP1 = 'UTC '
TCUNI1 = 'd '
TRPOS1 =
In [18]: t2 = Table.read('time.fits', astropy_native=True)
WARNING: VerifyWarning: Invalid keyword for column 1: Time coordinate reference position option (TRPOSn) must be a string (got <astropy.io.fits.card.Undefined object at 0x119711198>). The invalid keyword will be ignored for the purpose of formatting this column. [astropy.io.fits.column]
@aaryapatil ??