fix crash when no world is selected and configure button is pressed.

by moving return statement out of if-then-else clause...
Improve behaviour when no mods are installed:

Only show enable all / disable all buttons for all add-ons when at
least one add-on is installed. When no add-on ist installed, don't
show any buttons or checkboxes.
fix crash when pressing delete button in server browser and no server is selected

check for that was there, but was comparing an unsigned variable to -1, which doesn't work.
don't treat empty modpacks like normal mods in configuration GUI

added is_modpack flag to ModSpec to distinguish empty modpacks from
normal mods.