fluent-ui: Allow value of 0 in TextWidget

[pksorensen]

Fixes proper undefined/null check and not falsy check. when value is 0 its changed to "" instead of "0"

Reasons for making this change

[Please describe them here]

If this is related to existing tickets, include links to them as well. Use the syntax fixes #[issue number] (ex: fixes #123).

Checklist

• I'm updating documentation
  • I've checked the rendering of the Markdown text I've added
• I'm adding or updating code
  • I've added and/or updated tests
  • I've updated docs if needed
• I'm adding a new feature
  • I've updated the playground with an example use of the feature

[epicfaace]

Thanks, can you add a test?

[pksorensen]

I can try, not sure where to start.