Fix ValueError with unencrypted metadata values (Fixes #766). (#774)

* Fix crash with unencrypted metadata values (#766).

* Explicitly check for length

* Update CHANGELOG.md

Co-authored-by: Pieter Marsman <pietermarsman@gmail.com>

CHANGELOG.md

@@ -8,6 +8,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ### Fixed
 
+- `ValueError` when trying to decrypt empty metadata values ([#766](https://github.com/pdfminer/pdfminer.six/issues/766))
 - Sphinx errors during building of documentation ([#760](https://github.com/pdfminer/pdfminer.six/pull/760))
 - `TypeError` when getting default width of font ([#720](https://github.com/pdfminer/pdfminer.six/issues/720))
 

pdfminer/pdftypes.py

@@ -138,6 +138,8 @@ def resolve_all(x: object, default: object = None) -> Any:
 def decipher_all(decipher: DecipherCallable, objid: int, genno: int, x: object) -> Any:
     """Recursively deciphers the given object."""
     if isinstance(x, bytes):
+        if len(x) == 0:
+            return x
         return decipher(objid, genno, x)
     if isinstance(x, list):
         x = [decipher_all(decipher, objid, genno, v) for v in x]