refactor(inputters): Ensure inputter options are always a table

Annoyingly, options passed to an inputter constructor can be nil,
depending on how the inputter is invoked. Let's be more tolerant and
safe in that case, initializing to an empty table.

inputters/base.lua

@@ -12,7 +12,7 @@ inputter._name = "base"
 inputter._docclass = nil
 
 function inputter:_init (options)
-  if options then self.options = options end
+  self.options = options or {}
 end
 
 function inputter:classInit (options)