replace: tolerate None replacements

Avoids confusion/traceback when you leave off the replacement string instead
of using ''.

beets/ui/__init__.py

@@ -463,13 +463,15 @@ def get_replacements():
     """
     replacements = []
     for pattern, repl in config['replace'].get(dict).items():
+        repl = repl or ''
         try:
             replacements.append((re.compile(pattern), repl))
         except re.error:
             raise UserError(
                 u'malformed regular expression in replace: {0}'.format(
                     pattern
-            ))
+                )
+            )
     return replacements
 
 def get_plugin_paths():

docs/changelog.rst

@@ -63,11 +63,13 @@ Little fixes:
   Mathijs de Bruin.
 * The :ref:`clutter` option can now be a whitespace-separated list in addition
   to a YAML list.
-
+* Values for the :ref:`replace` option can now be empty (i.e., null is
+  equivalent to the empty string).
 
 .. _Acoustic Attributes: http://developer.echonest.com/acoustic-attributes.html
 .. _MPD: http://mpd.wikia.com/wiki/Music_Player_Daemon_Wiki
 
+
 1.3.1 (October 12, 2013)
 ------------------------