topological nodes set can not be null

[marcosmc]

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)

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

What is the current behavior? (You can also link to an open issue here)
When obtaining the list of topological nodes of a bus, it is verified that it is not null.

What is the new behavior (if this is a feature change)?
Currently this list cannot be null, so the check is unnecessary.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

95.5% Coverage
0.0% Duplication

[zamarrenolm]

I think we can merge this PR, but after reviewing it I think we need to improve the export at least in two ways:

• When doing a full export from a pure IIDM Network, the extension CgmesIidmMapping should also be created and filled. It contains mapping data between CGMES and IIDM that should be reused if the same Network is exported again (base voltages, for example). Other mapping information is already been stored as aliases and properties, so it is also reused if re-exporting.
• The mapping between Network buses and TNs should be "updated". Currently, it is either read from the extension or created from scratch. If a user imports a Network from CGMES, then creates new buses on the Network, the mapping would became invalid (would miss mappings for the new buses).

[miovd]

I think we can merge this PR, but after reviewing it I think we need to improve the export at least in two ways:

* When doing a full export from a pure IIDM Network, the extension CgmesIidmMapping should also be created and filled. It contains mapping data between CGMES and IIDM that should be reused if the same Network is exported again (base voltages, for example). Other mapping information is already been stored as aliases and properties, so it is also reused if re-exporting.

* The mapping between Network buses and TNs should be "updated". Currently, it is either read from the extension or created from scratch. If a user imports a Network from CGMES, then creates new buses on the Network, the mapping would became invalid (would miss mappings for the new buses).

I agree with you. I was already working on some improvements including making CgmesIidmMapping topology invalid when the network topology is modified.