Fix test failure in VectorSimilarityFunctionsIT (elastic#133742)

Small fix as randomVector can be null too here.

fixes elastic#133559

Author: iverase

x-pack/plugin/esql/src/internalClusterTest/java/org/elasticsearch/xpack/esql/vector/VectorSimilarityFunctionsIT.java

@@ -131,7 +131,7 @@ public void testSimilarityBetweenConstantVectorAndField() {
             valuesList.forEach(values -> {
                 float[] left = readVector((List<Float>) values.get(0));
                 Double similarity = (Double) values.get(1);
-                if (left == null) {
+                if (left == null || randomVector == null) {
                     assertNull(similarity);
                 } else {
                     assertNotNull(similarity);