PARQUET-7: [thrift] avoid thrift amender if all fields are optional.

No perf tests... I did some rudimentary testing which seems to bear out the idea but nothing scientific yet.
This is correct. since before it's saying "if (isRequired) return true;"
When it hits the default case, meaning it's a primitive type and also isRequired is false, so here it should just return false.
To make the intention of the code more clear, maybe @dvryaboy  can change it to "return isRequired"