Use nbsp in checkbox label if empty

To keep the label tag from having a 0 height (causing the checkbox to be mispositioned), some text needs to be in the label. This uses a nonbreaking space when checkbox_label is set to 'label'. See #898.
Merge branch 'patch-1' of github.com:kcivey/MopaBootstrapBundle into kcivey-patch-1
changed accordingly