Fix isHexString validation

[shuse2]

What was the problem?

This PR resolves #8531

How was it solved?

• Update validation logic to check length to be even

How was it tested?

• Added unit test

[codecov]

Codecov Report

Merging #8556 (09c20f3) into release/6.0.0 (8b8df42) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 09c20f3 differs from pull request most recent head 9551b19. Consider uploading reports for the commit 9551b19 to get more accurate results

Additional details and impacted files

@@              Coverage Diff               @@
##           release/6.0.0    #8556   +/-   ##
==============================================
  Coverage          83.33%   83.33%           
==============================================
  Files                593      593           
  Lines              22334    22335    +1     
  Branches            3277     3277           
==============================================
+ Hits               18612    18613    +1     
  Misses              3722     3722           

Impacted Files	Coverage Δ
elements/lisk-validator/src/validation.ts	98.21% <100.00%> (ø)

... and 3 files with indirect coverage changes