structure_cache in `mathics.core.expression.structure` is `None` but we try to set it in `_is_neutral_symbol()`

[rocky]

Description

Structure cache object is None, but we try to set it as though it were a dictionary in _is_neutral_symbol().

How to Reproduce and Output Given

$ mathics --post-mortem

In[1]:=  Needs["CodeParser`"] (* Note: You should have arranged to have CodeParser in your packages *)

ReplaceAll::reps: PacletInformation[CodeParser] is not a valid replacement rule.
ReplaceAll::reps: PacletInformation[CodeParser] is not a valid replacement rule.
ReplaceAll::reps: FileNameJoin[{Location /. PacletInformation[CodeParser], PacletInfo.wl}] is not a valid replacement rule.
CodeParser::notransport: No transport specified.

In[2]:= CodeTokenize["1"]
Traceback (most recent call last):
  File "/tmp/Mathics3/mathics-core/mathics/core/expression.py", line 1907, in _is_neutral_symbol
    definition = definitions.get_definition(symbol_name, only_if_exists=True)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/Mathics3/mathics-core/mathics/core/definitions.py", line 506, in get_definition
    raise KeyError
KeyError
...

Expected behavior

No error

Additional context

I will be putting in a PR to initialize cache to the empty dictionary instead of None, assuming that is what is intended.