Fixed #17529 -- get_template_from_string default arguments break

``get_template_from_string`` default arguments were breaking
``assertTemplateUsed``. The solution has been to return only the names of the
templates with a ``name`` attribute distinct of ``None``. The default ``name``
kwarg of ``Template`` has been changed to ``None``, more pythonic than ``'<Unknown
Template>'``.

tests/test_utils/views.py

@@ -1,5 +1,6 @@
 from django.http import HttpResponse
 from django.shortcuts import get_object_or_404
+from django.template import loader, Context
 
 from .models import Person
 
@@ -7,3 +8,7 @@ from .models import Person
 def get_person(request, pk):
     person = get_object_or_404(Person, pk=pk)
     return HttpResponse(person.name)
+
+def no_template_used(request):
+    template = loader.get_template_from_string("This is a string-based template")
+    return HttpResponse(template.render(Context({})))