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

## 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

```python
from monsterui.franken import Card
print(to_xml(Card("content", cls=None)))
# Renders:

# ^^^^^ 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:

```python
# 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`.

Start with the monsterui.franken.Card entry point and reproduce the issue using to_xml(Card("content", cls=None)). Trace the class-building path to find where None becomes a string; done means cls=None omits "None" and leaves the component's default classes, matching the behavior of an omitted cls argument.

Written by the indexing model from the issue text.