Enable filling null/missing values from dataset/heatmap

[nickofthyme]

Is your feature request related to a problem? Please describe.

An easy way to fill missign/null values from an arbitrary numeric dataset and heatmap cells.

Describe the solution you'd like

• Heatmap: A way to easily set a static fallback value when the value of a cell is missing. This behaviour would not be the default.
• General: A utility function to return missing data or fill missing values in a dataset. To be used on table and other places apart from the heatmap to fill values.

Describe alternatives you've considered
Handling fill logic manually in kibana.

Kibana Cross Issues

• [Lens] support dynamic coloring for missing values kibana#96404
• [Lens] Formula: provide fitting function for missing/null values kibana#115761
• Heatmap visualizations don't adjust color values for dark mode kibana#97636
• Full grid and "-" values on heatmap kibana#30681