Skip user `VaadinServlet`s without @WebServlet instead of NPE on `quarkusBuild`

[grossws]

Description

If VaadinServlet without @WebServlet annotation present (like when vaadin-cdi happened to be in classpath) currently quarkusBuild will fail with NPE.

This patch skips such servlets with a warning in log.

Fixes #76

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.
• [x] 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.
• [ ] New and existing tests are passing locally with my change.
• [x] I have performed self-review and corrected misspellings.

[CLAassistant]

All committers have signed the CLA.

[mcollovati]

@grossws thanks for the contribution! Do you mind signing the CLA?

[grossws]

@mcollovati, I've signed CLA, feel free to merge.

[sonarcloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

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