fix divide by zero bug (#447)

### What problem does this PR solve?

#445 

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

deepdoc/parser/pdf_parser.py

         """
 
     def __char_width(self, c):
-        return (c["x1"] - c["x0"]) // len(c["text"])
+        return (c["x1"] - c["x0"]) // max(len(c["text"]), 1)
 
     def __height(self, c):
         return c["bottom"] - c["top"]