DocumentPicker: Added value handling for legacy/migrated data

If the input `value` is a `string`, then we can handle that as an `string[]`

src/Umbraco.Web.UI.Client/src/packages/documents/documents/components/input-document/input-document.element.ts

@@ -163,7 +163,7 @@ export class UmbInputDocumentElement extends FormControlMixin(UmbLitElement) {
 	}
 
 	#renderItems() {
-		if (!this._items) return;
+		if (!this._items?.length) return;
 		return html`<uui-ref-list>
 			${repeat(
 				this._items,

src/Umbraco.Web.UI.Client/src/packages/documents/documents/property-editors/document-picker/property-editor-ui-document-picker.element.ts

@@ -13,7 +13,7 @@ export class UmbPropertyEditorUIDocumentPickerElement extends UmbLitElement impl
 		return this._value;
 	}
 	public set value(value: Array<string>) {
-		this._value = value || [];
+		this._value = Array.isArray(value) ? value : value ? [value] : [];
 	}
 
 	@property({ attribute: false })