Fixed handling of empty Time Freq Support

[ansys-akarcher]

These changes solve the #967 crash, when using fields_container_factory.over_time_freq_fields_container(...)

Edit: this code is wrong.
we should use a dict when we want a TimeFreqSupport
And min_max_* should not be dependent on TimeFreqSupport

[codecov]

Codecov Report

Merging #996 (1902de8) into master (13b21d6) will decrease coverage by 1.30%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master     #996      +/-   ##
==========================================
- Coverage   88.69%   87.39%   -1.30%     
==========================================
  Files          76       76              
  Lines        8623     8634      +11     
==========================================
- Hits         7648     7546     -102     
- Misses        975     1088     +113     

[ansys-akarcher]

I reverted as previously said,

Also the following code was enough to produce a crash:

fields_container = dpf.FieldsContainer()
print(fields_container.time_freq_support)

Traceback (most recent call last):
  File "C:\Users\akarcher\pydpf-core-venv\bug_minmax_overtime\test.py", line 16, in <module>
    print(fields_container.time_freq_support)
  File "C:\Users\akarcher\pydpf-core-venv\pydpf-core\src\ansys\dpf\core\fields_container.py", line 427, in time_freq_support
    return self._get_time_freq_support()
  File "C:\Users\akarcher\pydpf-core-venv\pydpf-core\src\ansys\dpf\core\collection.py", line 397, in _get_time_freq_support
    time_freq = support_api.support_get_as_time_freq_support(support)
  File "d:\ansysdev\ansys-dpf-pygate\ansys-dpf-gate\ansys\dpf\gate\generated\support_capi.py", line 71, in support_get_as_time_freq_support
    raise errors.DPFServerException(sError.value)
ansys.dpf.gate.errors.DPFServerException: Provided shared object is not a FieldSupport !

[ansys-akarcher]

@cbellot000 @anslpa The new test "test_fields_container_empty_tf_support()" doesn't work with LegacyGrpcServer, not sure on how to proceed from there, as the grpc interface returns an UNAVAILABLE when retrieving a nullptr (which makes sense)

[ansys-akarcher]

Completes #967