Fix AC voltage sensitivities

[Hadrien-Godard]

Signed-off-by: Hadrien hadrien.godard@artelys.com

Please check if the PR fulfills these requirements (please use '[x]' to check the checkboxes, or submit the PR and then click the checkboxes)

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Does this PR already have an issue describing the problem ? If so, link to this issue using '#XXX' and skip the rest
No

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Upgrade: Test whereas a voltage sensitivity from a PV bus which switched to PQ returns 0 or NaN.
New feature: Predefined results for voltage sensitivities when post contingency case breaks connectivity.

What is the current behavior? (You can also link to an open issue here)
Currently, voltage sensitivity from a PV bus which switched to PQ values 0.
Currently, there are no predefined results for voltage sensitivities when post contingency case breaks connectivity.

What is the new behavior (if this is a feature change)?
Predefined results for voltage sensitivities when post contingency case breaks connectivity.

Does this PR introduce a breaking change or deprecate an API? If yes, check the following:

• The Breaking Change or Deprecated label has been added
• The migration guide has been updated in the github wiki (What changes might users need to make in their application due to this PR?)

Other information:

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