`DateTimeField` does not handle empty values correctly

[mjparker777]

Reference:
#2869
#2687

This issue was fixed by 2869 for DateField and need the same fix for DateTimeField.

    def to_representation(self, value):
        if not value:
            return None

If I have a serializer with
start = serializers.DateTimeField(required=False)
then I should not have to supply a value but if I dont supply a value I get the below error.

2015-12-12 00:31:18,347 ERROR RequestDetailsHandler::get::371 
Traceback (most recent call last):
  File "/x/local/aitv2/automatic_integration_v2/eci/handlers/RequestDetailsHandler.py", line 368, in get
    return response.Response(RequestDetailsResponseSerializer(response_data).data,
  File "/Users/miparker/.virtualenvs/aitv3/lib/python2.7/site-packages/rest_framework/serializers.py", line 503, in data
    ret = super(Serializer, self).data
  File "/Users/miparker/.virtualenvs/aitv3/lib/python2.7/site-packages/rest_framework/serializers.py", line 239, in data
    self._data = self.to_representation(self.instance)
  File "/Users/miparker/.virtualenvs/aitv3/lib/python2.7/site-packages/rest_framework/serializers.py", line 472, in to_representation
    ret[field.field_name] = field.to_representation(attribute)
  File "/Users/miparker/.virtualenvs/aitv3/lib/python2.7/site-packages/rest_framework/serializers.py", line 614, in to_representation
    self.child.to_representation(item) for item in iterable
  File "/Users/miparker/.virtualenvs/aitv3/lib/python2.7/site-packages/rest_framework/serializers.py", line 472, in to_representation
    ret[field.field_name] = field.to_representation(attribute)
  File "/Users/miparker/.virtualenvs/aitv3/lib/python2.7/site-packages/rest_framework/serializers.py", line 472, in to_representation
    ret[field.field_name] = field.to_representation(attribute)
  File "/Users/miparker/.virtualenvs/aitv3/lib/python2.7/site-packages/rest_framework/fields.py", line 1068, in to_representation
    value = value.isoformat()
AttributeError: 'str' object has no attribute 'isoformat'

[mjparker777]

I made the needed changes and created a pull request. Includes outputs for tests.
#3731

[mjparker777]

All checks have passed, ready for review and merge.

[py2010]

django 2.2.*
rest_framework 3.9.1
multipart/form-data POST (have files upload)
DateTimeField "required": false,

from rest_framework import fields

def rewrite(cls, func='to_internal_value'):
    # 重写函数,使提交保存的日期时间字段为""时,不报格式错误
    old_func = getattr(cls, func)

    def new_func(self, value):
        # import ipdb; ipdb.set_trace()
        if value == '':
            # return self.get_default()  # 旧数据原值不变或新数据取默认值
            return None  # 表字段数据设置为null
        return old_func(self, value)
    setattr(cls, func, new_func)
    # print(cls, '已重写函数', func)

rewrite(fields.DateTimeField)
rewrite(fields.DateField)
rewrite(fields.TimeField)