tagString should ignore `null/undefined` substitutions

To allow something like


```javascript
function m (bar) {
  return ltx`<foo bar='${bar}'>`
}
m(undefined)
```

to return `<foo/>`

maybe even

```javascript
function m (bar) {
  return ltx`<foo><${bar}/></foo>`
}
m(undefined)
```

to return `<foo/>`