Devonshire

This was another empty-span-tag issue, caused by the combination of Cocoon and Firefox. Cocoon spits out empty tags as self-closing tags (which is wrong according to W3C guidelines), and Firefox screws up rendering when it encounters a self-closing tag which can have content, such as <span/> (which is annoying but at least draws our attention to the problem). In the case of this error, the empty span tags were caused by lines which have no number (i.e. no @n attribute), because they're in some way a refrain or a tagline that shouldn't be numbered. Fixed this by adding non-breaking spaces where there are no line numbers.