FIX(scanner): parameters can only be copied if present

src/caoscrawler/converters/converters.py

@@ -576,16 +576,17 @@ class Converter(object, metaclass=ABCMeta):
                                        " of the function!")
                 tr_func_key = list(tr_func_el.keys())[0]
 
-                # Create a copy of the function parameters:
-                tr_func_params = dict(tr_func_el[tr_func_key])
-
                 if tr_func_key not in transformer_functions:
                     raise RuntimeError("Unknown transformer function: {}".format(tr_func_key))
 
                 # Do variable replacment on function parameters:
-                if tr_func_params is not None:
+                if tr_func_el[tr_func_key] is not None:
+                    # Create a copy of the function parameters:
+                    tr_func_params = dict(tr_func_el[tr_func_key])
                     for key in tr_func_params:
                         tr_func_params[key] = replace_variables(tr_func_params[key], values)
+                else:
+                    tr_func_params = None
 
                 # Retrieve the function from the dictionary:
                 tr_func = transformer_functions[tr_func_key]