[Lens] Fix pie chart with 0 decimal places for percent

[wylieconlon]

Fixes #104207

The fix is to allow 0 as a valid debounced value. I can't come up with a way to test this fix because the debug data for pie charts only contains the input values, not the formatted output, so this appears to be manually tested only.

I did unskip the functional tests for pie chart since I thought it would help me in my testing, but that's included in this PR as a bonus test that isn't actually related to the core change.

Steps to verify:

1. Open a pie chart
2. Open the text icon and choose 0 for the percent formatter
3. The pie slices should display whole percents, not decimal

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: eb5910d
• Storybooks Preview

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
lens	1.5MB	1.5MB	+17.0B

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[stratoula]

Code LGTM, this works fine. I tested it locally for all partition charts. It works as expected

[kibanamachine]

💚 Backport successful

Status	Branch	Result
✅	7.14
✅	7.x

The backport PRs will be merged automatically after passing CI.