Accept that scalable can be empty + throw exception if a percentage is undefined in ProportionalScalable

[miovd]

Signed-off-by: VEDELAGO MIORA miora.ralambotiana@rte-france.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 but linked to powsybl/powsybl-entsoe#22

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Fixes:

• accepting empty scalable (linked to GLSK case)
• throwing an error when percentage is NaN

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

• 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?)

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[sonarqubecloud]

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