fix: Default to empty string if userAgent is empty

[silverchen]

Description

Bowser throws when userAgent is not a string. Default userAgent to empty string when it's empty fixes the issue.

Motivation and Context

This is to prevent non-browser app (React Native) from crashing.

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have added tests to cover my changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• My code follows the code style of this project.
• My commits and the title of this PR follow the Conventional Commits Specification.
• I have read the contributing guidelines.

[offirgolan]

Thank you for the PR @silverchen. Its been released under v2.6.1.