create_spoken_forms: If file_extensions.csv is empty, FILE_EXTENSIONS_REGEX is wrong

[rntz]

In create_spoken_forms.py we define FILE_EXTENSIONS_REGEX like so:

FILE_EXTENSIONS_REGEX = "|".join(
    re.escape(file_extension.strip()) + "$"
    for file_extension in file_extensions.values()
)

The problem with this is: what if file_extensions.values() is empty? Then we get "|".join([]) which is "". Unfortunately this is wrong. We want it to be the regex which matches nothing (no strings, not even the empty string). Instead it is the regex which matches the empty string, so it will match every zero-length position in a string.

To write the regex which matches nothing, you can do r"^\b$".

I can't follow how FILE_EXTENSIONS_REGEX is used in the rest of the file, so I'm not sure whether this could actually cause any problems. @pokey ?

(Noticed this while reviewing #1199.)