Made it so getNextPrompt() is called rather than acceptInput(context, null)

This allows for conversations to by a bit more dynamic, by giving developers the ability to specify which prompt should be used in the case that the conversation doesn't wait for input. If the developer allows for the conversation to sometimes accept input, then they would have to check for null input inside acceptsInput(). They should, however, be able to override getNextPrompt() so that when blocksForInput() returns false, the conversation doesn't pass null into acceptsInput() and instead uses getNextPrompt().