calendar: CalendarTooltip doesn't support valueFormat

[kamranayub]

Describe/explain the bug

When using Calendar or TimeRange, you can provide a valueFormat function:

(v: number) => string;

In CalendarTooltip.tsx, value is assumed to always be a number:

nivo/packages/calendar/src/CalendarTooltip.tsx

Line 6 in 2cf2243

if (value === undefined || isNaN(Number(value))) return null

This will return null when using a valueFormat function because it returns a string, not a number, which fails to show a tooltip.

To Reproduce

https://codesandbox.io/p/sandbox/mystifying-dhawan-4qhh8x

Steps to reproduce the behavior:

1. Go to csb
2. Hover or try to click on time range day
3. Observe no tooltip is shown

Expected behavior

A tooltip should be shown

Workaround

Copy original tooltip and make your own without the isNaN check