Don't export null altitude, but support importing null lat/lon/alt as 0s

[ianmcorvidae]

Fixes #548

This makes it so null lat/lon/alt are just treated as 0s if present, but also makes it so lat & lon won't export nulls and alt won't export at all unless it's set.

[codecov]

Codecov Report

Attention: Patch coverage is 57.14286% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 62.41%. Comparing base (bdf3a24) to head (4226201).

Files	Patch %	Lines
meshtastic/__main__.py	57.14%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #549      +/-   ##
==========================================
+ Coverage   62.39%   62.41%   +0.02%     
==========================================
  Files          14       14              
  Lines        2877     2879       +2     
==========================================
+ Hits         1795     1797       +2     
  Misses       1082     1082              

Flag	Coverage Δ
unittests	62.41% <57.14%> (+0.02%)	⬆️

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