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

1 files changed, 5 insertions, 1 deletions

diff --git a/py/makeqstrdata.py b/py/makeqstrdata.py
index 96e3956b4..b4f4f1b03 100644
--- a/py/makeqstrdata.py
+++ b/py/makeqstrdata.py
@@ -109,7 +109,11 @@ class TextSplitter:
     def __init__(self, words):
         words.sort(key=lambda x: len(x), reverse=True)
         self.words = set(words)
-        self.pat = re.compile("|".join(re.escape(w) for w in words) + "|.", flags=re.DOTALL)
+        if words:
+            pat = "|".join(re.escape(w) for w in words) + "|."
+        else:
+            pat = "."
+        self.pat = re.compile(pat, flags=re.DOTALL)
 
     def iter_words(self, text):
         s = []