[Java] Are the constraints of not-nullable MapVector necessary in Java?

[whutjs]

There are two not-nullable constraints in the Java implementation of MapVector:

1. Map data should be a non-nullable struct type; https://github.com/apache/arrow/blob/d4516c5386f84619dfdf2a9f72fed6d7df89704c/java/vector/src/main/java/org/apache/arrow/vector/complex/MapVector.java#L98
2. Map data key type should be a non-nullable; https://github.com/apache/arrow/blob/d4516c5386f84619dfdf2a9f72fed6d7df89704c/java/vector/src/main/java/org/apache/arrow/vector/complex/MapVector.java#L105

Are these constraints necessary? Because I cannot find these constraints in the Arrow Specifications .

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