Schema.Parser.validate should not be null

The class `org.apache.avro.Schema.Parser` has a private final property `validate`. This implies that should `null` be passed to the constructor, it is destined to throw an NPE at a later, arbitrary time when `validate` will be used.

Thus, I propose a simple one-line guard against `null` here for a fail-fast error, which would allow for a much easier debugging.