fix(VDataTable): set initial values for sort-desc and group-desc

[nekosaur]

Description

supplying sort-by and/or group-by values without corresponding sort-desc/group-by values could lead to issues when sorting/grouping

Motivation and Context

closes #9685

How Has This Been Tested?

playground, updated existing test

Markup:

<template>
  <v-container>
    <v-data-table
      :headers="headers"
      :items="desserts"
      item-key="name"
      group-by="category"
      :sort-by="sortBy"
      show-group-by
      multi-sort
    ></v-data-table>
  </v-container>
</template>

<script>
  export default {
    data () {
      return {
        sortBy: ['category', 'name'],
        headers: [
          { text: "Category", value: "category" },
          { text: "Dessert (100g serving)", align: "left", value: "name" },
          { text: "Calories", value: "calories" },
          { text: "Fat (g)", value: "fat" },
          { text: "Carbs (g)", value: "carbs" },
          { text: "Protein (g)", value: "protein" },
          { text: "Iron (%)", value: "iron" }
        ],
        desserts: [
          {
            name: "Frozen Yogurt",
            calories: 159,
            fat: 6.0,
            carbs: 24,
            protein: 4.0,
            iron: "1%",
            category: "Ice cream"
          },
          {
            name: "Ice cream sandwich",
            calories: 237,
            fat: 9.0,
            carbs: 37,
            protein: 4.3,
            iron: "1%",
            category: "Ice cream"
          },
          {
            name: "Eclair",
            calories: 262,
            fat: 16.0,
            carbs: 23,
            protein: 6.0,
            iron: "7%",
            category: "Cookie"
          },
          {
            name: "Cupcake",
            calories: 305,
            fat: 3.7,
            carbs: 67,
            protein: 4.3,
            iron: "8%",
            category: "Pastry"
          },
          {
            name: "Gingerbread",
            calories: 356,
            fat: 16.0,
            carbs: 49,
            protein: 3.9,
            iron: "16%",
            category: "Cookie"
          },
          {
            name: "Jelly bean",
            calories: 375,
            fat: 0.0,
            carbs: 94,
            protein: 0.0,
            iron: "0%",
            category: "Candy"
          },
          {
            name: "Lollipop",
            calories: 392,
            fat: 0.2,
            carbs: 98,
            protein: 0,
            iron: "2%",
            category: "Candy"
          },
          {
            name: "Honeycomb",
            calories: 408,
            fat: 3.2,
            carbs: 87,
            protein: 6.5,
            iron: "45%",
            category: "Toffee"
          },
          {
            name: "Donut",
            calories: 452,
            fat: 25.0,
            carbs: 51,
            protein: 4.9,
            iron: "22%",
            category: "Pastry"
          },
          {
            name: "KitKat",
            calories: 518,
            fat: 26.0,
            carbs: 65,
            protein: 7,
            iron: "6%",
            category: "Candy"
          }
        ]
      };
    }
  }
</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 features but makes 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 backwards compatible changes and next for non-backwards compatible 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)