Make sure verse blocks can contain empty lines

The previous verse parsing code made the faulty assumption that empty
strings are valid (and empty) inlines.  This isn't the case, so lines
are changed to contain at least a newline.

It would generally be nicer and faster to keep the newlines while
splitting the string.  However, this would require more code, which
seems unjustified for a simple (and fairly rare) block as *verse*.

This fixes #2402.

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