Minor fix in ForceAtlas2 conrad calls

Due to some weird schedule when using quickly stopForceAtlas2 / startForceAtlas2, it appears that sometimes the addJob function is called when the job has actually already been added. So, I just added a quick job check before adding it.
Improved quad behavior before indexation

I met some troubles sometimes calling the retrieve methods of the quad ("point" and "area") before the indexation, so I just initialized the _tree attribute as null when instanciated, and checked that _tree is not null before calling those methods. If its null, then the quad will now just return an empty array.