fix: return immidiately if VaadinSession is not initialized

[denis-anisimov]

fixes #11961

Description

Please list all relevant dependencies in this section and provide summary of the change, motivation and context.

Fixes # (issue)

Type of change

• Bugfix
• Feature

Checklist

• I have read the contribution guide: https://vaadin.com/docs/latest/guide/contributing/overview/
• 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.
• New and existing tests are passing locally with my change.
• 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.

[vaadin-bot]

SonarQube analysis reported 3 issues

1. VaadinSession.java#L210: This block of commented-out lines of code should be removed.
2. VaadinSession.java#L1077: Make the "java.util.Map" class "Serializable" or don't write it.
3. VaadinSession.java#L79: Do not forget to remove this deprecated code someday.

[vaadin-bot]

Hi @denis-anisimov and @caalador, when i performed cherry-pick to this commit to 8.0, i have encountered the following issue. Can you take a look and pick it manually?
Error Message:
Error: Command failed: git cherry-pick a7aac4b
error: could not apply a7aac4b... fix: return immidiately if VaadinSession is not initialized (#11974)
hint: after resolving the conflicts, mark the corrected paths
hint: with 'git add ' or 'git rm '
hint: and commit the result with 'git commit'

[vaadin-bot]

Hi @denis-anisimov and @caalador, when i performed cherry-pick to this commit to 2.7, i have encountered the following issue. Can you take a look and pick it manually?
Error Message:
Error: Command failed: git cherry-pick a7aac4b
error: could not apply a7aac4b... fix: return immidiately if VaadinSession is not initialized (#11974)
hint: after resolving the conflicts, mark the corrected paths
hint: with 'git add ' or 'git rm '
hint: and commit the result with 'git commit'

[vaadin-bot]

Hi @denis-anisimov and @caalador, when i performed cherry-pick to this commit to 7.0, i have encountered the following issue. Can you take a look and pick it manually?
Error Message:
Error: Command failed: git cherry-pick a7aac4b
error: could not apply a7aac4b... fix: return immidiately if VaadinSession is not initialized (#11974)
hint: after resolving the conflicts, mark the corrected paths
hint: with 'git add ' or 'git rm '
hint: and commit the result with 'git commit'