[Java] Write list of maps with the vector API

### Describe the usage question you have. Please include as many useful details as possible.

I'd like to store list of maps. Not sure if I am doing it wrong or there is a bug in the implementation.
Here is my example:
```java
try (ListVector listVector = ListVector.empty("sourceVector", allocator)) {

UnionListWriter listWriter = listVector.getWriter();

listWriter.allocate();

/*
* [{"one" -> 1, "two" -> 2}, {"three" -> 3}, null, {"four" -> null, "five" -> 5}]
*/

listWriter.setPosition(0);
listWriter.startList();

listWriter.map().startMap(); // <-- exception is thrown from here
listWriter.map().startEntry();
listWriter.map().key().varChar().writeVarChar("one");
listWriter.map().key().integer().writeInt(1);
listWriter.map().endEntry();
listWriter.map().startEntry();
listWriter.map().key().varChar().writeVarChar("two");
listWriter.map().key().integer().writeInt(2);
listWriter.map().endEntry();
listWriter.map().endMap();

listWriter.map().startMap();
listWriter.map().startEntry();
listWriter.map().key().varChar().writeVarChar("three");
listWriter.map().key().integer().writeInt(3);
listWriter.map().endEntry();
listWriter.map().endMap();

listWriter.writeNull();

listWriter.map().startMap();
listWriter.map().startEntry();
listWriter.map().key().varChar().writeVarChar("four");
listWriter.map().key().integer().writeNull();
listWriter.map().endEntry();
listWriter.map().startEntry();
listWriter.map().key().varChar().writeVarChar("five");
listWriter.map().key().integer().writeInt(5);
listWriter.map().endEntry();
listWriter.map().endMap();

listWriter.endList();
```

When I execute this I'm getting the following exception:
```
java.lang.UnsupportedOperationException: Cannot get simple type for type MAP
at org.apache.arrow.vector/org.apache.arrow.vector.types.Types$MinorType.getType(Types.java:807)
at org.apache.arrow.vector/org.apache.arrow.vector.complex.impl.PromotableWriter.getWriter(PromotableWriter.java:275)
at org.apache.arrow.vector/org.apache.arrow.vector.complex.impl.AbstractPromotableFieldWriter.getWriter(AbstractPromotableFieldWriter.java:80)
at org.apache.arrow.vector/org.apache.arrow.vector.complex.impl.AbstractPromotableFieldWriter.startMap(AbstractPromotableFieldWriter.java:114)
at org.apache.arrow.vector/org.apache.arrow.vector.complex.impl.PromotableWriter.startMap(PromotableWriter.java:53)
```

### Component(s)

Java

Reproduce el ejemplo de Java usando ListVector, UnionListWriter y las llamadas de escritura de mapas; después, inspecciona el stack trace de PromotableWriter donde falla startMap(). Determina si la vector API debería admitir este uso de listas de mapas; se considera completado cuando el ejemplo escribe los mapas indicados, la entrada de lista null y el valor de mapa null sin la excepción indicada.

Escrito por el modelo de indexación a partir del texto del issue.