fix(card): Don't write a units field for undefined values

[icweaver]

Hi Paul, I ran into this edge case while integrating FITSFiles.jl with AstroImages.jl. Would it make sense to drop the [] that sneaks into the comments section when the field is undefined?

Before

julia> Card("UNDEF", missing, "a comment")
UNDEF   =                      / [] a comment

After

julia> Card("UNDEF", missing, "a comment")
UNDEF   =                      / a comment

This makes handling empty fields easier downstream

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46.35%. Comparing base (8b68009) to head (2c31e5b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #43      +/-   ##
==========================================
+ Coverage   46.15%   46.35%   +0.19%     
==========================================
  Files          17       17              
  Lines        2550     2550              
==========================================
+ Hits         1177     1182       +5     
+ Misses       1373     1368       -5     

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[barrettp]

Do the brackets occur when reading (or parsing) a Card or when writing (or instantiating) a Card?

If the former, then the brackets are in the original FITS file and will not be modified. A basic requirement of FITSFiles is to maintain the original card format so that the written file is byte-for-byte as close to the read file as possible.

If the latter, then I'll take a look at it. Although that behaviour seems very unusual as I have not yet included support for units.

[icweaver]

It looks to be when writing, which then gets picked up again on read:

main

julia> using FITSFiles

julia> data = ones(Int32, (3,3));

julia> cards = [Card("KEY1", 3), Card("KEY2", missing)]
2-element Vector{Card}:
 KEY1    =                    3
 KEY2    =                      / []

julia> hdu = HDU(data, cards)
      Primary                               1       7     Int32   3 × 3

julia> write("output.fits", hdu)
2844

julia> fits("output.fits")[1].cards
7-element Vector{Card{<:Any}}:
 SIMPLE  =                    T
 BITPIX  =                   32
 NAXIS   =                    2
 NAXIS1  =                    3
 NAXIS2  =                    3
 KEY1    =                    3
 KEY2    =                      / []

This PR

julia> using FITSFiles

julia> data = ones(Int32, (3,3));

julia> cards = [Card("KEY1", 3), Card("KEY2", missing)]
2-element Vector{Card}:
 KEY1    =                    3
 KEY2    =

julia> hdu = HDU(data, cards)
      Primary                               1       7     Int32   3 × 3

julia> write("output.fits", hdu)
2844

julia> fits("output.fits")[1].cards
7-element Vector{Card{<:Any}}:
 SIMPLE  =                    T
 BITPIX  =                   32
 NAXIS   =                    2
 NAXIS1  =                    3
 NAXIS2  =                    3
 KEY1    =                    3
 KEY2    =

[barrettp]

Good catch. The error would probably be found once units are implemented properly.