fix(VNumberInput): fix inc/dec slots with split control variant

[SonTT19]

Description

fixes #20056

Markup:

<template>
  <v-app>
    <v-container>
      <v-number-input control-variant="default" />
      <v-number-input control-variant="default">
        <template #increment> inc </template>
        <template #decrement> dec </template>
      </v-number-input>

      <v-number-input control-variant="split" />
      <v-number-input control-variant="split">
        <template #increment> inc </template>
        <template #decrement> dec </template>
      </v-number-input>
      <v-number-input control-variant="stacked" />
      <v-number-input control-variant="stacked">
        <template #increment> inc </template>
        <template #decrement> dec </template>
      </v-number-input>
    </v-container>
  </v-app>
</template>