fix exception in pdf parser (#584)

### What problem does this PR solve?
#451 

### Type of change

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

deepdoc/parser/pdf_parser.py

                         continue
 
                     if re.match(r"[0-9]{2,3}/[0-9]{3}$", up["text"]) \
-                            or re.match(r"[0-9]{2,3}/[0-9]{3}$", down["text"]):
+                            or re.match(r"[0-9]{2,3}/[0-9]{3}$", down["text"]) \
+                            or not down["text"].strip():
                         i += 1
                         continue