Dont fail completely on no prep text.

[reisner]

If a file has no text after tokenizing, don't fail catastrophically. Print out a warning instead.

Previously:

Prepping temp//106.txt
Error: length(text) == 1 is not TRUE
>

Now:

Prepping temp//106.txt
Warning: No text to prep in temp//106.txt
Prepping temp//107.txt
...

This was failing on a file I was trying to run on that only contained emojis. These were removed by skip_word_none = TRUE in stri_split_boundaries. Rather than change that code, I figure a warning would be a better solution.