feat(QSelect): delete single mode value with backspace quasarframework#6703 (quasarframework#8149)

Same conditions as in case of multiple apply:
- inputValue mult be empty
- hideSelected must not be active

Author: pdanpdan

ui/src/components/select/QSelect.js

@@ -647,12 +647,15 @@ export default Vue.extend({
       // backspace
       if (
         e.keyCode === 8 &&
-        this.multiple === true &&
         this.hideSelected !== true &&
-        this.inputValue.length === 0 &&
-        Array.isArray(this.value)
+        this.inputValue.length === 0
       ) {
-        this.removeAtIndex(this.value.length - 1)
+        if (this.multiple === true && Array.isArray(this.value)) {
+          this.removeAtIndex(this.value.length - 1)
+        }
+        else if (this.multiple !== true && this.value !== null) {
+          this.$emit('input', null)
+        }
         return
       }