Don't blow up when asking for schema at non-existent path

spec/config-spec.coffee

@@ -1060,6 +1060,9 @@ describe "Config", ->
           type: 'integer'
           default: 12
 
+        expect(atom.config.getSchema('foo.baz')).toBeUndefined()
+        expect(atom.config.getSchema('foo.bar.anInt.baz')).toBeUndefined()
+
       it "respects the schema for scoped settings", ->
         schema =
           type: 'string'

src/config.coffee

@@ -703,7 +703,7 @@ class Config
     schema = @schema
     for key in keys
       break unless schema?
-      schema = schema.properties[key]
+      schema = schema.properties?[key]
     schema
 
   # Deprecated: Returns a new {Object} containing all of the global settings and