cls=None renders as literal string 'None' in HTML class attribute

[linguistic76]

Summary

Passing cls=None to MonsterUI components results in the literal string "None" appearing in the rendered HTML class attribute, rather than the attribute being omitted.

Reproduction

from monsterui.franken import Card
print(to_xml(Card("content", cls=None)))
# Renders: <div class="None uk-card ...">content</div>
#                   ^^^^^ literal "None"

Expected behaviour

When cls=None is passed, the cls argument should be treated as absent — the same as not passing cls at all. The class attribute should contain only the component's default classes.

Current workaround

Every wrapper in downstream code must guard against None explicitly:

# Required in every wrapper to avoid "None" appearing in class output
MCardBody(*c, cls=cls, **kwargs) if cls else MCardBody(*c, **kwargs)

This boilerplate is needed on every component wrapper and is easy to forget.

Suggested fix

Guard against None in the class-building path so cls=None is equivalent to cls="" or omitting cls.