fix(VMenu): skip elements with 0 or NaN z-index

[KaelWD]

fixes #5351

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 not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any feature but make things better)

Checklist:

• The PR title is no longer than 64 characters.
• The PR is submitted to the correct branch (master for bug fixes and documentation updates, dev for new features and breaking changes).
• My code follows the code style of this project.
• I've added relevant changes to the documentation (applies to new features and breaking changes in core library)

[codecov]

Codecov Report

Merging #7532 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #7532   +/-   ##
=======================================
  Coverage   85.43%   85.43%           
=======================================
  Files         298      298           
  Lines        7221     7221           
  Branches     1804     1804           
=======================================
  Hits         6169     6169           
  Misses        956      956           
  Partials       96       96

Impacted Files	Coverage Δ
packages/vuetify/src/util/helpers.ts	90.84% <100%> (ø)	⬆️

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