Fixed an error when the textbox of NumberBaseConverter is empty.

[niyari]

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• UI change (please include screenshot!)
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Internationalization and localization
• Other (please describe):

What is the current behavior?

Issue Number: N/A

What is the new behavior?

• Fixed a bug in NumberBaseConverter that caused DevToys to quit when switching focus when the text box was empty.

Other information

Quality check

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• Followed the code style guideline as described in CONTRIBUTING.md
• Verified that the change work in Release build configuration
• Checked all unit tests pass

[veler]

Thank you for this! :)