AdminReadonlyField accepts the optional argument model_admin but treats it as non-optional

[Josh]

Using a FieldSet without providing a model_admin results in an error since AdminReadonlyField calls: model_admin.get_empty_value_display().

My expected behaviour is that it does not error, and sets the empty_value_display to the empty string.
Relevant file: ​https://github.com/django/django/blob/master/django/contrib/admin/helpers.py

Cheers.

[Tim Graham]

Duplicate of #27157.