Allow an empty config value

e.g. this enables to configure

gitlab.default_priority =

bugwarrior/services/__init__.py

@@ -2,6 +2,7 @@
 from builtins import str
 from builtins import object
 
+import ast
 import copy
 import multiprocessing
 import time
@@ -73,11 +74,14 @@ def __init__(self, main_config, main_section, target):
         log.info("Working on [%s]", self.target)
 
 
-    def _get_config_or_default(self, key, default, as_type=lambda x: x):
+    def _get_config_or_default(self, key, default, as_type=None):
         """Return a main config value, or default if it does not exist."""
 
         if self.main_config.has_option(self.main_section, key):
-            return as_type(self.main_config.get(self.main_section, key))
+            if as_type:
+                return as_type(self.main_config.get(self.main_section, key))
+            else:
+                return ast.literal_eval(self.main_config.get(self.main_section, key))
         return default