FieldType for random tests should return null for custom type

[parkertimmins]

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.

For example,

elasticsearch/test/framework/src/main/java/org/elasticsearch/datageneration/datasource/DefaultFieldDataGeneratorHandler.java

Line 17 in 5e2b199

if (fieldType == null) {

expects a null type when a custom type name is passed in.

Fixes #133202

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)