Fix bug in PushQueriesIT (#127421)

When the length of the String `value` is 0, test cases get
stuck in an infinite loop trying to select a different random
String with length 0. This commit changes it to select a String with
length 1 in this case.

Author: limotova

x-pack/plugin/esql/qa/server/single-node/src/javaRestTest/java/org/elasticsearch/xpack/esql/qa/single_node/PushQueriesIT.java

@@ -90,7 +90,7 @@ public void testPushCaseInsensitiveEqualityOnDefaults() throws IOException {
     private void testPushQuery(String value, String esqlQuery, String luceneQuery, boolean filterInCompute, boolean found)
         throws IOException {
         indexValue(value);
-        String differentValue = randomValueOtherThan(value, () -> randomAlphaOfLength(value.length()));
+        String differentValue = randomValueOtherThan(value, () -> randomAlphaOfLength(value.length() == 0 ? 1 : value.length()));
 
         String replacedQuery = esqlQuery.replaceAll("%value", value).replaceAll("%different_value", differentValue);
         RestEsqlTestCase.RequestObjectBuilder builder = requestObjectBuilder().query(replacedQuery + "\n| KEEP test");