Do not call str.length if str is undefined

Fixes #5599.

The problem is that `stringToBytes` in `src/shared/util.js` can be called with an `undefined` string for some PDFs. This PR makes sure we handle that case correctly by setting the length to 0. We issue a warning in that case to indicate that there might be something wrong with the PDF or with the code. Note that this function has to return a `Uint8Array`, even if it's of length 0, because other functionality in PDF.js depends on that.
@CodingFabian You are right. I thought about that before, but I didn't check how often it was called so I went for this solution. For http://www.etsi.org/deliver/etsi_ts/151000_151099/15101001/10.03.00_60/ts_15101001v100300p.pdf, a large PDF document, it is called almost 100 times per page. I will change that, thanks!