[docs] Added example of styling cells with note about not targeting empty cells

[wilcoschoneveld]

Follow-up for #18190

• I have followed (at least) the PR section of the contributing guide.

[github-actions]

Please add one type label to categorize the purpose of this PR appropriately:

bug, regression, enhancement, new feature, release or dependencies

[mui-bot]

Deploy preview: https://deploy-preview-18201--material-ui-x.netlify.app/

Updated pages:

• docs/data/data-grid/style/style.md

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Details of bundle changes

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[wilcoschoneveld]

@mapache-salvaje I have updated the text and demo to better indicate when you should exclude the emptyCell class from the styling selector