fix(ripple): allow getComputedStyle to return null

[neelance]

The function window.getComputedStyle was returning null under certain conditions on Firefox before version 62, see https://developer.mozilla.org/en-US/docs/Web/API/Window/getComputedStyle#Browser_compatibility.

Motivation and Context

JS Error on Firefox.

How Has This Been Tested?

Bug only happens on old Firefox versions. Is this something you want to explicitly test?

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 #6639 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #6639      +/-   ##
==========================================
+ Coverage   85.68%   85.68%   +<.01%     
==========================================
  Files         298      298              
  Lines        7174     7175       +1     
  Branches     1784     1784              
==========================================
+ Hits         6147     6148       +1     
  Misses        936      936              
  Partials       91       91

Impacted Files	Coverage Δ
packages/vuetify/src/directives/ripple.ts	75.78% <100%> (+0.19%)	⬆️

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[lock]

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