Fix exception for connectable not found in Network Modification connection and disconnection

[rolnico]

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)
• [ ] Docs have been added / updated (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 you use for example a ConnectableConnection on a non-existing connectable or on a TieLine (which is not a Connectable), the application of the NetworkModification will crash due to a null pointer exception

What is the new behavior (if this is a feature change)? You will now get a PowsyblException if you apply with throwException==true or the Network Modification will not be applied and the error will be logged

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

• [ ] Yes
• [X] No

[rolnico]

https://github.com/powsybl/powsybl-core/pull/3072 could also be another solution

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud