fix: disabled attribute fallback (#1984)

Co-authored-by: Dominik Pschenitschni <mail@celement.de>
Reviewed-on: https://kolaente.dev/vikunja/frontend/pulls/1984
Reviewed-by: konrad <k@knt.li>
Co-authored-by: Dominik Pschenitschni <dpschen@noreply.kolaente.de>
Co-committed-by: Dominik Pschenitschni <dpschen@noreply.kolaente.de>

src/components/tasks/edit-task.vue

@@ -11,7 +11,7 @@
 			<div class="control">
 				<input
 					:class="{ disabled: taskService.loading }"
-					:disabled="taskService.loading || null"
+					:disabled="taskService.loading || undefined"
 					@change="editTaskSubmit()"
 					class="input"
 					id="tasktext"

src/components/tasks/partials/attachments.vue

@@ -8,7 +8,7 @@
 		</h3>
 
 		<input
-			:disabled="attachmentService.loading || null"
+			:disabled="attachmentService.loading || undefined"
 			@change="uploadNewAttachment()"
 			id="files"
 			multiple

src/components/tasks/partials/defer-task.vue

@@ -30,7 +30,7 @@
 		<flat-pickr
 			:class="{ disabled: taskService.loading }"
 			:config="flatPickerConfig"
-			:disabled="taskService.loading || null"
+			:disabled="taskService.loading || undefined"
 			class="input"
 			v-model="dueDate"
 		/>