fix(VSelect): allow null values to be selected

[KaelWD]

Description

fixes #4431

Clearable still needs work, if you clear the select it'll revert to null and select the first item again.
No idea what those original checks were there for, it seems to work fine and tests still pass.
The internalValue === '' check was for #4032, but that's a separate component now and doesn't seem to have regressed.

Markup:

<template>
  <v-app>
    <v-container>
      <v-autocomplete
          v-model="selected"
          :items="items"
          label="Standard"
          clearable
      ></v-autocomplete>
      <pre>{{ JSON.stringify(selected) }}</pre>
    </v-container>
  </v-app>
</template>

<script>
  export default {
    data: () => ({
      selected: null,
      items: [
        { text: 'all of them', value: null },
        { text: 'fizz', value: 'Fizz' },
        { text: 'buzz', value: 'Buzz' },
        { text: 'foo', value: 'Foo' }
      ]
    })
  }
</script>

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)

[codecov]

Codecov Report

Merging #4649 into master will decrease coverage by 0.15%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4649      +/-   ##
==========================================
- Coverage   90.14%   89.98%   -0.16%     
==========================================
  Files         206      206              
  Lines        4919     4965      +46     
  Branches     1249     1274      +25     
==========================================
+ Hits         4434     4468      +34     
- Misses        388      394       +6     
- Partials       97      103       +6

Impacted Files	Coverage Δ
src/components/VSelect/VSelect.js	95.21% <ø> (ø)	⬆️
src/components/VAutocomplete/VAutocomplete.js	98.19% <100%> (-0.04%)	⬇️
src/components/VFooter/VFooter.js	100% <0%> (ø)	⬆️
.../components/VNavigationDrawer/VNavigationDrawer.js	70.14% <0%> (+2.33%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 098d07b...e66fbd4. Read the comment docs.

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs. Please direct any non-bug questions to our Discord