Fix validator for empty lists (the default value)

Whoops.

overviewer_core/settingsValidators.py

@@ -167,6 +167,7 @@ def validateOptImg(optimizers):
         raise ValidationException("What you passed to optimizeimg is not a list. "\
                                   "Make sure you specify them like [foo()], with square brackets.")
 
+    if optimizers:
         for opt, next_opt in zip(optimizers, optimizers[1:]) + [(optimizers[-1], None)]:
             if not isinstance(opt, Optimizer):
                 raise ValidationException("Invalid Optimizer!")