[Java] Minor types don't account for nullable FieldType flag

Calling e.g. `FLOAT4.getNewVector("foo", new FieldType(false, ...), ...)" returns a NullableFloat4Vector instead of a Float4Vector.
edit: Float4Vector doesn't implement FieldVector, so can't currently be a top-level vector. I'm confused as to what the nullable flag is supposed to represent then.

**Reporter**: [Emilio Lahr-Vivaz](https://issues.apache.org/jira/browse/ARROW-999) / @elahrvivaz

_{**Note**: *This issue was originally created as [ARROW-999](https://issues.apache.org/jira/browse/ARROW-999). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start at the Java implementation of FLOAT4.getNewVector and inspect how FieldType(false, ...) is interpreted for Float4Vector and NullableFloat4Vector. Trace the FieldVector constraint mentioned in the issue and determine the intended meaning of the nullable flag; done means the returned vector type and top-level vector behavior match that contract.

Written by the indexing model from the issue text.