Add check to ensure that ListSerializer is used only with a serializer as its child

[sevdog]

In #9386 it was asked to normalize the return value of ListSerializer.to_representation to better handle null values.

By reading the docs it states that:

The ListSerializer class provides the behavior for serializing and validating multiple objects at once. You won't typically need to use ListSerializer directly, but should instead simply pass many=True when instantiating a serializer.

In my intendement the use-case of providing a "normal" field as child to ListSerializer is not taken into account. To handle such use case it already exists the ListField.

I tried working on a PR to enforce this check and found out that there are some tests where a ListSerializer is istantiated with a field as child in the unittests:

django-rest-framework/tests/test_serializer_lists.py

Lines 218 to 220 in 4d06626

	class TestSerializer(serializers.Serializer):
	integers = serializers.ListSerializer(child=serializers.IntegerField())
	booleans = serializers.ListSerializer(child=serializers.BooleanField())

django-rest-framework/tests/test_serializer_lists.py

Lines 340 to 351 in 4d06626

	class TestSerializer(serializers.Serializer):
	integers = serializers.ListSerializer(
	child=serializers.ListSerializer(
	child=serializers.IntegerField()
	)
	)
	booleans = serializers.ListSerializer(
	child=serializers.ListSerializer(
	child=serializers.BooleanField()
	)
	)

django-rest-framework/tests/schemas/test_coreapi.py

Line 68 in 4d06626

b = serializers.ListSerializer(child=serializers.CharField())

django-rest-framework/tests/test_metadata.py

Lines 79 to 83 in 4d06626

	list_field = serializers.ListField(
	child=serializers.ListField(
	child=serializers.IntegerField()
	)
	)

My point is: it is fine to let ListSerializer have any kind of child or it would be better to have it validating the type of its child?