fix(ActionInsertLink): Set href default to empty string instead of null

With @nextcloud/vue 8.5.0, NcInputField requires a string or number as
`value` property.

Signed-off-by: Jonas <jonas@freesources.org>

src/components/Menu/ActionInsertLink.vue

@@ -110,7 +110,7 @@ export default {
 	],
 	data: () => {
 		return {
-			href: null,
+			href: '',
 			isInputMode: false,
 			startPath: null,
 			/** Open state of the actions menu */
@@ -170,7 +170,7 @@ export default {
 				const href = [...event.target.elements].filter(e => e?.type === 'text')[0].value
 				this.menuOpen = false
 				this.isInputMode = false
-				this.href = null
+				this.href = ''
 				return this.setLink(href, href)
 			}