fix(VDataTable): no multiple select if last entry not selected

[florealcab]

Description

If the last selected item is no more selected, we should not reselect it and all items between if we press "shift" and click on another item. Instead, ignore the shift behavior in this case.

fixes #13684

Motivation and Context

#13684

How Has This Been Tested?

There was no test for this feature (or I missed them), so I don't add unit tests. I test with my real example and it fixes the problem.

Markup:

// Paste your FULL Playground.vue here
<template>
  <v-app>
    <v-container>
      <v-data-table
              :headers="table_headers"
              :items="table_data"
              item-key="name"
              show-select
      ></v-data-table>
    </v-container>
  </v-app>
</template>
<script>
  module.exports = {
        data: function() {
            return {
                table_data: [
                    {
                        "name": 1,
                        "column1": "Toto",
                        "column2": "Tutu"
                    },
                    {
                        "name": 2,
                        "column1": "Toto",
                        "column2": "Tutu"
                    },
                    {
                        "name": 3,
                        "column1": "Toto",
                        "column2": "Tutu"
                    },
                    {
                        "name": 4,
                        "column1": "Toto",
                        "column2": "Tutu"
                    }
                ],
                table_headers: [
                        {align: "left", text: "Column 1", value: "column1"},
                        {align: "left", text: "Column 2", value: "column2"}
                ]
            }
        }
    }
</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)

[KaelWD]

This seems to remove shift key deselection entirely.

• Select all items
• Deselect the second one
• Hold shift and deselect the fourth

I'd expect 2,3,4 to be deselected, but now only 2 and 4 are.

[florealcab]

Oh yes, I fix that. Now if last entry is selected, shift cause select of items between, else shift cause unselect of them.