add > to s:IsAnEmptyListItem regex

Adding `>` to the `s:IsAnEmptyListItem` regular expression character list seems to fix the issue where hitting enter on an empty `>` previously did not auto-end the quote.

Here is a demo with the new optional setting (`g:markdown_drop_empty_blockquotes`):

![](https://cloudup.com/cZoXO6vwoV7+)