Heatmap axis tooltip shows even when no value is present

[nickofthyme]

Describe the issue

The Heatmap tooltip has a unique function used to show tick value labels on hover. This is used by ML to enable showing long axis labels in their full un-truncated form.

The issue is when the value is empty the tooltip is shown anyway, leaving a long skinny white tooltip.

Expected behaviour
The empty tooltip should be hidden.