Fixed #24045 -- Removed useless mark_safe() call in trans_null.py

from django.conf import settings

from django.utils.encoding import force_text

from django.utils.safestring import mark_safe, SafeData

def ngettext(singular, plural, number):

def gettext(message):

    if isinstance(message, SafeData):

        return mark_safe(message)

    return message