fix: handling of empty values in NcDateTimePickerNative

Signed-off-by: Richard Steinmetz <richard@steinmetz.cloud>

src/components/NcDateTimePickerNative/NcDateTimePickerNative.vue

@@ -2,6 +2,7 @@
   - @copyright Copyright (c) 2022 Julia Kirschenheuter <julia.kirschenheuter@nextcloud.com>
   -
   - @author Julia Kirschenheuter <julia.kirschenheuter@nextcloud.com>
+  - @author Richard Steinmetz <richard@steinmetz.cloud>
   -
   - @license GNU AGPL version 3 or any later version
   -
@@ -33,13 +34,16 @@ All available types are: 'date', 'datetime-local', 'month', 'time' and 'week', p
 #### Usage: type='datetime-local'
 ```vue
 <template>
-	<span>
-		<NcDateTimePickerNative
-			v-model="value"
-			:id="id"
-			:label="label"
-			type="datetime-local" />
-	</span>
+	<div>
+		<span>
+			<span>Picked date: {{ valueString }}</span>
+			<NcDateTimePickerNative
+				v-model="value"
+				:id="id"
+				:label="label"
+				type="datetime-local" />
+		</span>
+	</div>
 </template>
 <script>
 	export default {
@@ -50,6 +54,14 @@ All available types are: 'date', 'datetime-local', 'month', 'time' and 'week', p
 				label: 'please select a new date',
 			}
 		},
+		computed: {
+			valueString() {
+				if (this.value === null) {
+					return 'null'
+				}
+				return this.value.toString()
+			}
+		}
 	}
 </script>
 ```
@@ -163,10 +175,11 @@ export default {
 		 * The date is – like the `Date` object in JavaScript – tied to UTC.
 		 * The selected time zone does not have an influence of the selected time and date value.
 		 * You have to translate the time yourself when you want to factor in time zones.
+		 * Pass null to clear the input field.
 		 */
 		value: {
 			type: Date,
-			required: true,
+			default: null,
 		},
 
 		/**
@@ -265,7 +278,7 @@ export default {
 						 *
 						 * @return {string} empty string
 						 */
-						return this.$emit('input', '')
+						return this.$emit('input', null)
 					}
 					if (this.type === 'time') {
 						const time = $event.target.value