fix: fixes on user servlet registrations

[mcollovati]

Description

@WebServlet.urlPatterns are read as string array from annotation. @WebServlet.value is also taken into accout for mappings. Added check to prevent NPE when reading init parameters. Use @WebServlet.name as servlet name, if present.

Type of change

• [X] Bugfix
• [ ] Feature

Checklist

• [X] I have read the contribution guide: https://vaadin.com/docs/latest/guide/contributing/overview/
• [X] I have added a description following the guideline.
• [ ] The issue is created in the corresponding repository and I have referenced it.
• [ ] I have added tests to ensure my change is effective and works as intended.
• [X] New and existing tests are passing locally with my change.
• [X] I have performed self-review and corrected misspellings.

Additional for Feature type of change

• [ ] Enhancement / new feature was discussed in a corresponding GitHub issue and Acceptance Criteria were created.