ESQL: Fix DateDiff flaky test configuration (#138877)

Fixes #138772

The randomConfiguration() function generates a random query text unless it receives a explicit one. That random query makes the Source deserialization fail.

Author: ivancea

x-pack/plugin/esql/qa/testFixtures/src/main/java/org/elasticsearch/xpack/esql/ConfigurationTestUtils.java

@@ -51,6 +51,10 @@ public static Configuration randomConfiguration() {
         return randomConfiguration(randomRealisticUnicodeOfLength(len), randomTables());
     }
 
+    public static Configuration randomConfiguration(String query) {
+        return randomConfiguration(query, randomTables());
+    }
+
     public static Configuration randomConfiguration(String query, Map<String, Map<String, Column>> tables) {
         var zoneId = randomZone();
         var locale = randomLocale(random());

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/expression/function/scalar/date/DateDiffTests.java

@@ -152,7 +152,7 @@ private static List<TestCaseSupplier> makeSuppliers(
         Instant endTimestamp = Instant.parse(endTimestampString);
 
         Supplier<Configuration> configurationSupplier = () -> timezone == null
-            ? randomConfiguration()
+            ? randomConfiguration(TEST_SOURCE.text())
             : configurationForTimezone(ZoneId.of(timezone));
 
         return Stream.of(DataType.KEYWORD, DataType.TEXT)