[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.*}

從 FLOAT4.getNewVector 的 Java 實作開始，檢查 FieldType(false, ...) 對 Float4Vector 和 NullableFloat4Vector 的解釋方式。追蹤 issue 中提到的 FieldVector 約束，並確定 nullable 旗標的預期含義；當回傳的向量型別和頂層向量行為符合該契約時，即表示完成。

由索引模型根據 Issue 內容生成。