ESQL: Fix unlikely error in suggested cast tests (#127567)

The suggested casts tests would error out less than one percent of the
time because they tried to suggest a cast for `unsupported` and
expected `keyword`. That'll always be null.

Caught this in an unreleated PR.

Author: nik9000

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/type/EsqlDataTypeConverterTests.java

@@ -199,7 +199,7 @@ public void testSuggestedCast() {
             Set<DataType> typesToTest = new HashSet<>(Set.of(DATETIME, DATE_NANOS));
             assertEquals(DATE_NANOS, DataType.suggestedCast(typesToTest));
 
-            DataType randomType = DataType.values()[random().nextInt(DataType.values().length)];
+            DataType randomType = randomValueOtherThan(UNSUPPORTED, () -> randomFrom(DataType.values()));
             typesToTest.add(randomType);
             DataType suggested = DataType.suggestedCast(typesToTest);
             if (randomType != DATETIME && randomType != DATE_NANOS) {