FIX: robustness for emty fields in matlab models

[codecov[bot]]

Codecov Report

Merging #16 into master will increase coverage by 0.46%. The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #16      +/-   ##
=========================================
+ Coverage   64.94%   65.4%   +0.46%     
=========================================
  Files          36      36              
  Lines        2305    2330      +25     
=========================================
+ Hits         1497    1524      +27     
+ Misses        808     806       -2

Impacted Files	Coverage Δ
pytfa/io/base.py	90.65% <100%> (+1.91%)	:arrow_up:
tests/test_io.py	100% <100%> (ø)	:arrow_up:

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[psalvy]

Thanks for this @weilandtd :)