fix: drop event if beforeSend returns null

[josketres]

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] The commit message follows Conventional Commits
• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• [x] Bugfix
• [ ] Feature
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Other... Please describe:

What is the current behavior?

What is the new behavior?

The sentry javascript SDK allows event filtering by returning null from the beforeSend callback. See https://docs.sentry.io/platforms/javascript/configuration/filtering/

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information

[github-actions[bot]]

:tada: This pull request is included in version core@2.1.1 :tada:

The release is available on GitHub release

[github-actions[bot]]

:tada: This pull request is included in version browser@2.1.1 :tada:

The release is available on GitHub release

[github-actions[bot]]

:tada: This pull request is included in version angular@2.1.1 :tada:

The release is available on GitHub release

[IKatsuba]

@josketres, thank you for your contribution!