FEAT: set thermal modifier and spatial modifier to None

[Alberto-DM]

• added the possibility to set thermal modifier and spatial modifier to None
• fix: when setting the thermalmodifier and spatialmodifier properties, the material was changed properly, but PyAEDT property did not show the change.
• fix: if the thermal or spatial modifier was set to None (EITHER in the UI or in PyAEDT), further changes in the property modifiers would be ignored.

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Codecov Report

Attention: Patch coverage is 66.66667% with 12 lines in your changes missing coverage. Please review.

Project coverage is 83.18%. Comparing base (f541b56) to head (d899570).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #4908 +/- ## ========================================== - Coverage 83.18% 83.18% -0.01% ========================================== Files 118 118 Lines 54147 54177 +30 ========================================== + Hits 45041 45065 +24 - Misses 9106 9112 +6 ```