Make iOS attributes not reappear in empty lines

[szydlovsky]

This PR fixes two pretty obscure iOS issues;

• backspacing a line until it's empty made attributes from previous line be applied in the empty line. It looked especially dumb with lists/blockquotes that just got put there together with their zero width spaces (so custom drawing as well)
• just changing selection to an empty line that follows some non empty line also used the previous line's attributes. This wasn't as bad because custom drawing from lists/quotes wasn't there, but the indents from them still were there.