Fix tags missing when imported during collection

(cherry picked from commit 06d4dd8)

Author: izquierdo

django_coverage_plugin/plugin.py

@@ -78,6 +78,8 @@ def check_debug():
         return False
     if not hasattr(django.template.backends.django, "DjangoTemplates"):
         raise DjangoTemplatePluginException("Can't use non-Django templates.")
+    if not django.template.engines._engines:
+        return False
 
     for engine in django.template.engines.all():
         if not isinstance(engine, django.template.backends.django.DjangoTemplates):