text: Title: ImageField inside Maybe declaration no longer working since 3.1.0
  Body: #### Description
  In a factory that I defined for companies, I'm randomly generating a logo using a `Maybe` declaration. This used to work fine up to and including 3.0.1, but as of 3.1.0 it has different behaviour.
  
  #### To Reproduce
  
  ##### Model / Factory code
  Leaving out the other fields as they cannot be relevant to the problem.
  ```python
  from factory import Faker, Maybe
  from factory.django import DjangoModelFactory, ImageField
  from ..models import Company
  
  class CompanyFactory(DjangoModelFactory):
      logo_add = Faker("pybool")
      logo = Maybe(
          "logo_add",
          yes_declaration=ImageField(width=500, height=200, color=Faker("color")),
          no_declaration=None,
      )
  
      class Meta:
          model = Company
          exclude = ("logo_add",)
  ```
  
  ##### The issue
  Up to and including 3.0.1 the behaviour - which is the desired behaviour as far as I'm concerend - was that I could generate companies that either had a logo or did not (about 50/50 since I'm just using "pybool" for the decider field). If they had a logo, the logo would be 500x200 with a random color.
  
  Now that I use 3.1.0, the randomness of about half the companies having logos still works, but _all_ generated logo's are now 100x100 and blue, which are simply defaults (although the [documentation](https://factoryboy.readthedocs.io/en/latest/orms.html?highlight=imagefield#factory.django.ImageField) says that "green" is actually the default), which is definitely something to fix :)
  
  Perhaps I was misusing/misunderstanding this feature all along, but then I'd still like to know how to get the desired behaviour described.
  
  