Check if metadata attribute is not None before serialization

[registerrier]

Description

This pull request solves issue #5075 .
Before serializing a specific metadata attribute to a set of header keywords, the to_header() method now checks that it is not None.

Dear reviewer

[codecov]

Codecov Report

Attention: 14 lines in your changes are missing coverage. Please review.

Comparison is base (b5fee6f) 75.00% compared to head (4f30e80) 75.00%.
Report is 7 commits behind head on main.

❗ Current head 4f30e80 differs from pull request most recent head d877e7a. Consider uploading reports for the commit d877e7a to get more accurate results

Files	Patch %	Lines
gammapy/modeling/models/temporal.py	22.22%	7 Missing ⚠️
gammapy/modeling/models/tests/test_temporal.py	22.22%	7 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5076   +/-   ##
=======================================
  Coverage   75.00%   75.00%           
=======================================
  Files         232      232           
  Lines       34554    34554           
=======================================
  Hits        25917    25917           
  Misses       8637     8637           

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[bkhelifi]

Thanks a lot @registerrier