makeqstrdata: Work around python3.6 compatibility problem

Discord user Folknology encountered a problem building with Python 3.6.9,
`TypeError: ord() expected a character, but string of length 0 found`.

I was able to reproduce the problem using Python3.5*, and discovered that
the meaning of the regular expression `"|."` had changed in 3.7.  Before,
```
>>> [m.group(0) for m in re.finditer("|.", "hello")]
['', '', '', '', '', '']
```
After:
```
>>> [m.group(0) for m in re.finditer("|.", "hello")]
['', 'h', '', 'e', '', 'l', '', 'l', '', 'o', '']
```
Check if `words` is empty and if so use `"."` as the regular expression
instead.  This gives the same result on both versions:
```
['h', 'e', 'l', 'l', 'o']
```
and fixes the generation of the huffman dictionary.

Folknology verified that this fix worked for them.

 * I could easily install 3.5 but not 3.6.  3.5 reproduced the same problem

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