Bugfixes to getChatHistory API method

Fixes two issues:

When called without `start` and `end` arguments on a pad *without* any chat history, this api error is (I believe) incorrectly returned: "start is higher or equal to the current chatHead." Instead, it should return an empty array.

When called without `start` and `end` arguments on a pad *with* chat history, the most recent chat message is not returned. Instead, they should all be returned.