MapArray::new_from_strings creates nullable entries field

[wjones127]

Describe the bug

This line creates a nullable entries field:

arrow-rs/arrow-array/src/array/map_array.rs

Lines 330 to 334 in 878217b

	Arc::new(Field::new(
	"entries",
	entry_struct.data_type().clone(),
	true,
	)),

The Arrow spec dictates this should not be nullable:

https://github.com/apache/arrow/blob/c4b01c60fba85bbfc3a1b1510e179153c0f79515/format/Schema.fbs#L124

To Reproduce

Expected behavior

Creates a compliant map array.

Additional context