Fix error for multi cluster tests, where k can already have been rewritten

Author: carlosdelest

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/vector/Knn.java

@@ -192,6 +192,11 @@ private TypeResolution resolveQuery() {
     }
 
     private TypeResolution resolveK() {
+        if (k == null) {
+            // Function has already been rewritten and included in QueryBuilder - otherwise parsing would have failed
+            return TypeResolution.TYPE_RESOLVED;
+        }
+
         return isType(k(), dt -> dt == INTEGER, sourceText(), THIRD, "integer").and(isFoldable(k(), sourceText(), THIRD))
             .and(isNotNull(k(), sourceText(), THIRD));
     }