Add boolean type to empty class

[rodolvbg]

I think it would be a good idea to add a bool method to the empty class to be able to use it in the following cases.

django-rest-framework/rest_framework/fields.py

Lines 49 to 56 in 4bbfa8d

	class empty:
	"""
	This class is used to represent no data being provided for a given input
	or output value.
	It is required because `None` may be a valid input or output value.
	"""
	pass

Use cases:

data = empty
if not data: # this is like say -> if data in [None, "", False, empty]
  pass

[rodolvbg]

#9121