fix: allow null in setUser method

[sevaru]

PR Checklist

Please check if your PR fulfills the following requirements:

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

PR Type

Allow setUser to accept null

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

What is the current behavior?

There is no easy way to clear user

Closes #21

What is the new behavior?

setUser with null would allow to clear user

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information

[taiga-family-bot]

:tada: This pull request is included in version core@7.0.3 :tada:

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[taiga-family-bot]

:tada: This pull request is included in version browser@7.0.4 :tada:

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[taiga-family-bot]

:tada: This pull request is included in version angular@7.0.3 :tada:

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[taiga-family-bot]

:tada: This pull request is included in version breadcrumbs-plugin@1.0.1 :tada:

The release is available on GitHub release