Simplify FieldPanel kwarg handling
Simplify FieldPanel kwarg handling

there's no reason to bend over backwards to distinguish between not passing a kwarg and passing None...

wagtail/wagtail,thenewguy/wagtail,zerolab/wagtail,zerolab/wagtail,wagtail/wagtail,zerolab/wagtail,thenewguy/wagtail,wagtail/wagtail,rsalmaso/wagtail,rsalmaso/wagtail,wagtail/wagtail,rsalmaso/wagtail,zerolab/wagtail,rsalmaso/wagtail,thenewguy/wagtail,wagtail/wagtail,rsalmaso/wagtail,thenewguy/wagtail,thenewguy/wagtail,zerolab/wagtail