Fix updating of fk_main_wastewater_node/fk_main_cover

[cymed]

When creating the functions tww_app.wastewater_structure_update_fk_main_wastewater_node / tww_app.wastewater_structure_update_fk_main_cover, the idea was to call them only in obj_id mode if a structure has no fk_main_wastewater_node/fk_main_cover.

Now, we intend to use them in trigger functions regardless whether the fk_main_wastewater_node/fk_main_cover is NULL or not. This poses a problem as the function sets the fk_main_xxx to the min value of all cover/node oids linked to the wastewater structure. If I have a main cover that has a higher OID than a secondary one, the triggers will overwrite that setting on wastewater structure when altering any cover attribute.

To mitigate that problem,when calling tww_app.wastewater_structure_update_fk_main_xxx(obj_id) we now only update the main cover/wastewater_node if the value is NULL. Calling tww_app.wastewater_structure_update_fk_main_xxx(NULL) continues to set it for all where fk_main_xxx is NULL, calling tww_app.wastewater_structure_update_fk_main_xxx(NULL,true) overwrites the fk_main_xx for all entries