Avoid TypeError parsing a property name containing '@'

[eeshsaxena]

A property name containing @ makes cssutils raise a bare TypeError instead of a CSS error:

import cssutils
cssutils.css.CSSStyleDeclaration(cssText='col@or: red')
# TypeError: 'NoneType' object does not support item assignment  (util.py, ATKEYWORD)

The default productions added by _adddefaultproductions record the wellformed state with new['wellformed'] = False, but _parse's new argument is optional and defaults to None. Parsing a bare property name passes new=None, so when the name contains @ the ATKEYWORD production dereferences None. I default new to a throwaway dict at the top of _parse when it is None, so the malformed name is reported as a normal CSS error (dropped when parsing a whole sheet, SyntaxErr when building a declaration directly). Valid CSS is unaffected.

Added a test to TestCSSStyleDeclaration; it raises TypeError on main and passes with the change, and the declaration suite still passes. Found it by fuzzing parseString.