general: consider null variable value as None (#49146)

hobo/environment/models.py

@@ -77,7 +77,7 @@ class Variable(models.Model):
     def _parse_value_as_json(self):
         if self.value and (
                 self.value[0] in '{['
-                or self.value in ('true', 'false')
+                or self.value in ('true', 'false', 'null')
                 or FLOAT_RE.match(self.value)):
             try:
                 return json.loads(self.value)