AmplNetworkWriter: only call getExtension for the ones that have a writer

[jonenst]

Please check if the PR fulfills these requirements

• 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?

NO

What kind of change does this PR introduce?

Feature/bugfix

Does this PR introduce a new Powsybl Action implying to be implemented in simulators or pypowsybl?

• Yes, the corresponding issue is here
• No

What is the current behavior?

when exporting a network in ampl format, we call getExtensions and then we filter to do nothing based on the absence/presence of extension specific writes.

What is the new behavior (if this is a feature change)?
when exporting a network in ampl format, we only call getExtension for data that we will actually serialize

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

• Yes
• No

Other information:
This is very useful for iidm implementations like network-store which don't have everything loaded in memory and do network requests to load the data

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
92.3% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[flo-dup]

Suggested change

	List<AmplExtension> extList = extensionMap.computeIfAbsent(ext.getName(), k -> new ArrayList<>());
	extList.add(new AmplExtension(extendedNum, extendable, ext));
	extensionMap.put(ext.getName(), extList);
	extensionMap.computeIfAbsent(ext.getName(), k -> new ArrayList<>()).add(new AmplExtension(extendedNum, extendable, ext));