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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.16%. Comparing base (fd7c1c9) to head (d37b41c).
Report is 1 commits behind head on 4.x.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##                4.x     #593   +/-   ##
=========================================
  Coverage     95.16%   95.16%           
  Complexity      584      584           
=========================================
  Files            70       70           
  Lines          1840     1840           
=========================================
  Hits           1751     1751           
  Misses           89       89           

Files with missing lines	Coverage Δ
src/Type/Hexadecimal.php	92.00% <ø> (ø)

[ramsey]

Thank you for contributing! 🎉