Skip formatting machinery in text nodes without format args

[anp]

https://github.com/anp/moxie/blob/main/dom/src/interfaces/node.rs#L65

https://doc.rust-lang.org/stable/std/fmt/struct.Arguments.html#method.as_str is stable as of 1.53.

[zetanumbers]

So um, this method is useful if format_args! has no args. Btw i think we could use .to_string() or thread local buffer for this.