Pressing enter in a list with no nearby content creates a paragraph

Currently we assume getNearestContentOffset will give us something
sane however it can return -1 when there is no nearby content and so
an exception is thrown. In this case we have to create an empty
paragraph to place the cursor in.

Change-Id: Ic6c19da881e47ff6be45cdaa4b71bfcc1c654796
Merge "Pressing enter in a list with no nearby content creates a paragraph"