Update validation.md

[machineghost]

Clarified that the component requires a null value for the validation to treat it as acceptable. As I discovered, a blank string makes the component go into the invalid validation state ... but there's currently no explanation of this behavior in the docs (that I could find).

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

[mui-bot]

Deploy preview: https://deploy-preview-13014--material-ui-x.netlify.app/

Updated pages:

• docs/data/date-pickers/validation/validation.md

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