lark.grammar.Symbol.__eq__ raises an AssertionError when checking if None == Symbol(...)

[weaversam8]

Describe the bug

When attempting to compare a variable set to None with a Symbol, Lark's implementation of __eq__ within the Symbol class raises an AssertionError.

To Reproduce

from lark.grammar import Symbol
a = None
b = Symbol("abc")

# this throws an error:
a == b

The error message:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "[...]/venv/lib/python3.8/site-packages/lark/grammar.py", line 19, in __eq__
    assert isinstance(other, Symbol), other
AssertionError: None

[erezsh]

This class is mainly for internal use, but I will accept a PR that returns False if the other object is None.