fix(VBtn): disabled state background-color

[ElijahKotyluk]

add check to isElevated to ensure elevation prop is not null.

Description

fixes #12845

Motivation and Context

How Has This Been Tested?

playground

Markup:

<template>
  <div class="ma-12 pa-12">
    <v-card>
      <v-card-text>
        <v-container>
          <v-row
            align="center"
            justify="space-around"
            class="my-10"
          >
            <v-btn disabled>Disabled</v-btn>

            <v-btn color="red" :disabled="disabled"  outlined>outlined</v-btn>

            <v-btn color="primary"  :disabled="disabled" icon>
              <v-icon dark>
                mdi-vuetify
              </v-icon>
            </v-btn>

            <v-btn color="blue" :disabled="disabled" text>text</v-btn>

            <v-btn color="green" :disabled="disabled" elevation="5">elevated</v-btn>
          </v-row>

          <v-row
            align="center"
            justify="space-around"
            class="my-10"
          >
            <v-btn color="green"  :disabled="disabled"  plain>plain</v-btn>

            <v-btn color="red" depressed :disabled="disabled">depressed</v-btn>

            <v-btn color="blue" :disabled="disabled" rounded>rounded</v-btn>

            <v-btn color="primary" :disabled="disabled" tile>tile</v-btn>
          </v-row>
        </v-container>
      </v-card-text>
    </v-card>

    <v-row
      aligh="center"
      justify="center"
      class="my-10"
    >
      <v-btn color="black" @click="toggleDisabled">toggle</v-btn>
    </v-row>
  </div>
</template>
<script>
  export default {
    data () {
      return {
        disabled: false,
      }
    },
    methods: {
      toggleDisabled() {
        this.disabled = !this.disabled
      }
    },
  }
</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)