Module Finder - only show module description if .description key exists
Module Finder - only show module description if .description key exists

We only add the `description` to a Module object if its field in the
auxiliary module representation in AUXMODULES global is not -1.
This allows us to use Exhibit 3.0's `data-ex-if-exists` to conditionally
render the module description div.

Signed-off-by: Pang Yan Han <22594c345857f664c61cd411f607c4d40a9d4af9@gmail.com>

yanhan/cljs-nusmods,yanhan/cljs-nusmods,yanhan/cljs-nusmods