Bug: Make `add` command safe for null values

It seems like the `add` command does not accept null values, as in, I cannot say `add [@attribute=true] to `

Here's my example case: I have this hyperscript statement in a behavior

```hyperscript
add [@type="button"] to in me
```

In HTML, if this behavior is called on a node that does not contain any buttons, this breaks, complaining that ` in me` evaluates to null.

I think there are already other workarounds to use for this, for instance, the `tell` command should handle this better. But it would be nice for `add` to handle this gracefully, too.

Comienza localizando la implementación del comando `add` y las pruebas de los comandos que reciben valores null. Reproduce el comportamiento mostrado con un destino `` ausente, compáralo con el tratamiento del comando `tell` y confirma que `add` ya no falla cuando su destino se evalúa como null.

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