Add tests to empty simple fieldlist

[sandorkertesz]

Description

The PR:

• simplifies the values, to_numpy, to_array and data FieldList code for empty SimpleFieldLists
• adds test for this use-case

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.58333% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.99%. Comparing base (2c57091) to head (42f9f00).
⚠️ Report is 227 commits behind head on develop.

Files with missing lines	Patch %	Lines
tests/core/test_fieldlist.py	89.58%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #806      +/-   ##
===========================================
+ Coverage    85.98%   85.99%   +0.01%     
===========================================
  Files          174      175       +1     
  Lines        13467    13515      +48     
  Branches       676      676              
===========================================
+ Hits         11579    11622      +43     
- Misses        1698     1703       +5     
  Partials       190      190              

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