fix: closes #11343, don't crash if tags array is empty

src/database/postgres/sorted/union.js

@@ -32,6 +32,9 @@ SELECT COUNT(DISTINCT z."value") c
 
 	async function getSortedSetUnion(params) {
 		const { sets } = params;
+		if (!sets || !sets.length) {
+			return [];
+		}
 		const start = params.hasOwnProperty('start') ? params.start : 0;
 		const stop = params.hasOwnProperty('stop') ? params.stop : -1;
 		let weights = params.weights || [];