Security Analysis: Fix Generator initialTargetP in OperatorStrategies

[jeandemanged]

Please check if the PR fulfills these requirements

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

Does this PR already have an issue describing the problem?

No

What kind of change does this PR introduce?

Bug fix

What is the current behavior?

If there is a single operator strategy for a contingency, the operator strategy simulation is done using initialTargetP of the basecase state instead of the post-contingency state. Note: No issue if there are multiple operator strategies for a continency, in this case all operator strategies simulations are done using initialTargetP of the post-contingency state, as it should be. But there is a bug in the case of a single unique operator strategy for the contingency.

What is the new behavior (if this is a feature change)? Operator strategies correctly start from the good starting point always.

Does this PR introduce a breaking change or deprecate an API?

• [x] No

Other information:

Fix of https://github.com/powsybl/powsybl-open-loadflow/pull/1041