Desktop: Fix JEX export (#3026)

* Return empty default for exports with no note ids (jex for example)

* switch to ! syntax instead of == null

ElectronClient/InteropServiceHelper.js

@@ -96,6 +96,10 @@ class InteropServiceHelper {
 	}
 
 	static async defaultFilename(noteIds, fileExtension) {
+		if (!noteIds) {
+			return '';
+		}
+
 		const note = await Note.load(noteIds[0]);
 		// In a rare case the passed not will be null, use the id for filename
 		if (note === null) {