Improve appearance of vacant tile slots

[drewdaemon]

Is your feature request related to a problem? Please describe.
In some cases, the vacant tile slots can make the visualization appear broken.

Describe the solution you'd like
Clearer visual queues that these slots are empty for a reason instead of just being broken.

Describe alternatives you've considered
Nothing here...

Additional context
Can look at @MichaelMarcialis's review for more context. We have received similar feedback from others as well.

Checklist

• this request is checked against already exist requests

[drewdaemon]

Relevant discussion / ideas elastic/kibana#137769 (comment)

[markov00]

cc @gvnmagni can you post here the final design of those empty tiles?

[gvnmagni]

Here you can find the screenshots, I'm not reposting the same images here in order to avoid repetition but please let me know if you prefer to have them also here

elastic/kibana#137769