fix crash when tap on an empty UITextView

Well, in this case, I think we should have to check UITextView is not empty at first.
Otherwise, this will causes an exception:
`'NSRangeException', reason: 'NSRLEArray objectAtIndex:effectiveRange:: Out of bounds'`
Rendering single page markdown correctly

A problem occurred in the rendered output whenever the body of
content was smaller than a single page (basically, the content
fit inside of the visible bounds of the markdown view).

This fixes that problem.