undefined no more

[leastbad]

Many operations will place the word undefined instead of an empty string if the critial parameter is omitted.

1. Nobody is looking for a way to emit undefined. This is not a feature.
2. There are many examples of operations where not specifying the critical parameter could be a feature if used creatively. For example, this PR would make it possible to:

cable_ready.inner_html(selector: "#foo")

This would erase the contents of the element.

Admittedly, I have already done this to the set_value operation before I realized that >50% of the operations would be better-suited to an empty string default.