new repl prompt should not fail on an empty cancellation line

[jurgenvinju]

Describe the bug

rascal>x = 1
│2 >>>> 
❌ Rascal could not recognize this command, beyond line 2 and column 0
rascal>
│2 >>>>
Cancelled
rascal>

The documentation around the ContinuationPrompt fails on the above example. If the last line of a command is empty the error message "❌ Rascal could not recognize this command, beyond line 2 and column 0" should not be printed. Instead it should print "Cancelled" like the next command does.

This is blocking the release of the website; for good reasons. I miss the old behavior personally too.