Fixed #18400 -- Modified length template filter to return 0 for unknown variables.

Thanks Florian for the bug report, luyikei for the initial code patch, and
Bouke for the code review feedback.

django/template/defaultfilters.py

@@ -578,7 +578,7 @@ def length(value):
     try:
         return len(value)
     except (ValueError, TypeError):
-        return ''
+        return 0
 
 
 @register.filter(is_safe=False)