Strict checks skip check for `undefined`

This broke here:
https://github.com/johndyer/mediaelement/commit/9304e51fc52561acbf44341671796e919b5cbda3#diff-c828f76ddf61bb6b8160755ebf3de782R970

Ultimately, you want to not set the poster if the value of `posterUrl` is falsely. Previously, `undefined` didn't make it through because `undefined == null` equals `true` but `undefined === null` equals `false`. In the 2nd case, `posterUrl` is only checked for empty string and `null`, but the function call is derived from, `player.$media.attr('poster')`, returns `undefined`.