FieldType for random tests should return null for custom type (elastic#133271)

The data generation framework allows the use of custom types. When attempting 
to parse the type name into a FieldType, this should return null, which will cause 
downstream users to skip this type.

Fixes elastic#133202

Co-authored-by: Alan Woodward <[email protected]>

Author: parkertimmins

test/framework/src/main/java/org/elasticsearch/datageneration/FieldType.java

@@ -108,7 +108,8 @@ public static FieldType tryParse(String name) {
             case "wildcard" -> FieldType.WILDCARD;
             case "passthrough" -> FieldType.PASSTHROUGH;
             case "match_only_text" -> FieldType.MATCH_ONLY_TEXT;
-            default -> throw new IllegalArgumentException("Unknown field type: " + name);
+            // Custom types will fail to parse and will return null
+            default -> null;
         };
     }