fix: allow None for addplot and alines kwargs in plot()

[Jo2234]

Summary

Fix addplot and alines kwarg validators to accept None, matching their documented default values.

Problem

When users conditionally build plot arguments and explicitly pass addplot=None or alines=None, a TypeError is raised because the validators don't accept None:

# This raises TypeError even though None is the documented default
mpf.plot(df, type='candle', volume=True, addplot=None, alines=None)

TypeError: kwarg "addplot" validator returned False for value: "None"
TypeError: kwarg "alines" validator returned False for value: "None"

This is a common pattern when conditionally building kwargs:

adp = build_addplots()  # may return None
tradeline = build_lines()  # may return None
mpf.plot(df, addplot=adp, alines=tradeline)  # crashes if either is None

Fix

-  'Validator': lambda value: isinstance(value,dict) or (isinstance(value,list) and all([isinstance(d,dict) for d in value]))
+  'Validator': lambda value: value is None or isinstance(value,dict) or (isinstance(value,list) and all([isinstance(d,dict) for d in value]))

-  'Validator': lambda value: _alines_validator(value)
+  'Validator': lambda value: value is None or _alines_validator(value)

Both kwargs already have 'Default': None and the plot() function body already checks for None (lines 421, 659, 740), so this fix makes the validators consistent.

Fixes #681