Hexadecimal is never an empty string

[axlon]

Description

Similarly to UUIDs, a hexadecimal can never be empty, this PR documents it as such.

Motivation and context

More and more libraries are starting to enforce stricter typing, I was dealing with a library that required a non-empty-string which hexadecimal isn't (is, but isn't documented that it is) which requires either baselining false positives are adding redundant assertions.

How has this been tested?

This PR contains no functional changes, I've tested the changes through PHPStan.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

PR checklist

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING.md document.
• I have added tests to cover my changes.