Model conversion ICRS <-> ECL works if PM uncertainties are not set

[dlakaplan]

Previously if the PM uncertainties were None, model.as_ICRS() would fail. This fixes that.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.01 🎉

Comparison is base (d5ea5e8) 68.12% compared to head (c89724a) 68.13%.

❗ Current head c89724a differs from pull request most recent head 0ecdba9. Consider uploading reports for the commit 0ecdba9 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1598      +/-   ##
==========================================
+ Coverage   68.12%   68.13%   +0.01%     
==========================================
  Files          98       98              
  Lines       22621    22619       -2     
  Branches     3883     3883              
==========================================
+ Hits        15410    15411       +1     
+ Misses       6261     6260       -1     
+ Partials      950      948       -2     

Impacted Files	Coverage Δ
src/pint/models/astrometry.py	92.20% <ø> (ø)

... and 6 files with indirect coverage changes

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