[EuiFilterButton] Allow for zero notifications

[pickypg]

Summary

The existing check implicitly disallows 0, which causes the rendered 0 to lack the proper styling.

versus with this fix

Checklist

• A changelog entry exists and is marked appropriately
• This was checked for breaking changes and labeled appropriately
• Jest tests were updated or added to match the most common scenarios

Workaround

To anyone using it prior to this fix, you could supply numFilters as numFilters={value || undefined} to avoid the awkward 0.

[cchaos]

The numFilters != null check should probably be checked and stored early on in the component, because there are classes applied that rely on that logic as well

eui/src/components/filter_group/filter_button.js

Line 46 in 053a7da

{ 'euiFilterButton__text-hasNotification': numFilters, },

[pickypg]

Good catch @cchaos! Updated to share the boolean check across both usages.

[cchaos]

Thanks!