fix(VDatePicker): display proper year when model is null

[jacekkarczmarczyk]

Motivation and Context

fixes #5663

How Has This Been Tested?

visually, unit

Markup:

<template>
  <v-app>
    <v-content>
      <v-date-picker
        v-model="date1"
      ></v-date-picker>
      <v-date-picker
        v-model="date2"
      ></v-date-picker>
      <v-date-picker
        v-model="date1"
        :picker-date.sync="pickerDate"
      ></v-date-picker>
      <v-date-picker
        v-model="date2"
        :picker-date.sync="pickerDate"
      ></v-date-picker>
    </v-content>
  </v-app>
</template>

<script>
export default {
  data: () => ({
    date1: null,
    date2: '2015-05',
    pickerDate: '2013-05'
  })
}
</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)

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)

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[codecov]

Codecov Report

Merging #5672 into master will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5672      +/-   ##
==========================================
- Coverage   89.05%   88.99%   -0.06%     
==========================================
  Files         262      262              
  Lines        5922     5909      -13     
  Branches     1467     1468       +1     
==========================================
- Hits         5274     5259      -15     
+ Misses        531      530       -1     
- Partials      117      120       +3

Impacted Files	Coverage Δ
.../vuetify/src/components/VDatePicker/VDatePicker.js	92.06% <ø> (ø)	⬆️
...fy/src/components/transitions/expand-transition.js	50% <0%> (-14%)	⬇️
...etify/src/components/VMenu/mixins/menu-position.js	50% <0%> (-5.56%)	⬇️
packages/vuetify/src/components/VMenu/VMenu.js	87.5% <0%> (-2.7%)	⬇️
packages/vuetify/src/components/VDialog/VDialog.js	72.58% <0%> (-0.15%)	⬇️
packages/vuetify/src/mixins/menuable.js	95.19% <0%> (-0.1%)	⬇️
packages/vuetify/src/components/VSelect/VSelect.js	88.42% <0%> (-0.06%)	⬇️
packages/vuetify/src/mixins/overlayable.js	47.19% <0%> (ø)	⬆️
...ify/src/components/VTimePicker/VTimePickerClock.js	100% <0%> (ø)	⬆️
packages/vuetify/src/mixins/delayable.ts	100% <0%> (ø)	⬆️
... and 2 more

---

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[codecov]

Codecov Report

Merging #5672 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5672      +/-   ##
==========================================
- Coverage   89.05%   89.04%   -0.02%     
==========================================
  Files         262      262              
  Lines        5922     5922              
  Branches     1467     1467              
==========================================
- Hits         5274     5273       -1     
- Misses        531      532       +1     
  Partials      117      117

Impacted Files	Coverage Δ
.../vuetify/src/components/VDatePicker/VDatePicker.js	92.06% <ø> (ø)	⬆️
...fy/src/components/transitions/expand-transition.js	60% <0%> (-4%)	⬇️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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