config: Add optional support to Boolean type

docs/changelog.rst

@@ -31,6 +31,10 @@ Feature release.
   release, like Mopidy 0.18, or migrate the configuration to the new format by
   hand.
 
+**Configuration**
+
+- Add ``optional=True`` support to :class:`mopidy.config.Boolean`.
+
 **Logging**
 
 - Fix proper decoding of exception messages that depends on the user's locale.

mopidy/config/types.py

@@ -151,7 +151,13 @@ class Boolean(ConfigValue):
     true_values = ('1', 'yes', 'true', 'on')
     false_values = ('0', 'no', 'false', 'off')
 
+    def __init__(self, optional=False):
+        self._required = not optional
+
     def deserialize(self, value):
+        validators.validate_required(value, self._required)
+        if not value:
+            return None
         if value.lower() in self.true_values:
             return True
         elif value.lower() in self.false_values:

tests/config/test_types.py

@@ -214,6 +214,10 @@ class BooleanTest(unittest.TestCase):
         self.assertEqual(b'false', result)
         self.assertIsInstance(result, bytes)
 
+    def test_deserialize_respects_optional(self):
+        value = types.Boolean(optional=True)
+        self.assertEqual(None, value.deserialize(''))
+
     # TODO: test None or other invalid values into serialize?