Course urls may be null when navigation is set to true.
The return value from course_get_url being null was not being handled before
this change, which could lead to an exception causing the page not to load.
It also handles null urls in the course index in Boost so that they are
not a link to what ever the current page is.
If either the first or last section in a format did not a
return a url under the Classic theme there would still be
a link displayed with a link to the current page the user
is on.
After this change no link would be displayed in this case.
Neill Magill
ee5126a88e