avoid shredding empty files, also x/0

also cleaned up unused import and exception var

lib/ansible/parsing/vault/__init__.py

@@ -71,7 +71,7 @@ try:
 except ImportError:
     pass
 
-from ansible.compat.six import PY3, byte2int
+from ansible.compat.six import PY3
 from ansible.utils.unicode import to_unicode, to_bytes
 
 HAS_ANY_PBKDF2HMAC = HAS_PBKDF2 or HAS_PBKDF2HMAC
@@ -236,6 +236,8 @@ class VaultEditor:
         """
 
         file_len = os.path.getsize(tmp_path)
+
+        if file_len > 0: # avoid work when file was empty
             max_chunk_len = min(1024*1024*2, file_len)
 
             passes = 3
@@ -273,7 +275,7 @@ class VaultEditor:
 
         try:
             r = call(['shred', tmp_path])
-        except OSError as e:
+        except OSError:
             # shred is not available on this system, or some other error occured.
             r = 1