[C] SchemaSetType(MAP) should set entries/keys as non-nullable

[lidavidm]

From apache/arrow-adbc#453

We should set it here (since the default is to make things nullable):

arrow-nanoarrow/src/nanoarrow/schema.c

Lines 143 to 155 in 5771e5e

	case NANOARROW_TYPE_MAP:
	NANOARROW_RETURN_NOT_OK(ArrowSchemaAllocateChildren(schema, 1));
	NANOARROW_RETURN_NOT_OK(
	ArrowSchemaInitFromType(schema->children[0], NANOARROW_TYPE_STRUCT));
	NANOARROW_RETURN_NOT_OK(ArrowSchemaSetName(schema->children[0], "entries"));
	NANOARROW_RETURN_NOT_OK(ArrowSchemaAllocateChildren(schema->children[0], 2));
	ArrowSchemaInit(schema->children[0]->children[0]);
	ArrowSchemaInit(schema->children[0]->children[1]);
	NANOARROW_RETURN_NOT_OK(
	ArrowSchemaSetName(schema->children[0]->children[0], "key"));
	NANOARROW_RETURN_NOT_OK(
	ArrowSchemaSetName(schema->children[0]->children[1], "value"));
	break;

SchemaViewValidateMap should also validate this