fix($compile): correctly handle `null`/`undefined` href `attrs.$set()`

Accidentally broken while backporting #14890.

Since #14890, `$$sanitizeUri()` can no longer handle `null`/`undefined` values.
In 1.7.x, there are no such calls.
In 1.6.x, there is still one such calls inside `Attributes.$set()`, so it needs to be adjusted accordingly.

Closes #16520