How to add javascript library that does not need a module, but is required by modules?
A module is really the only appropriate place for something like that; Drupal doesn't provide any way of making a module dependant on a JS library. You can only make a module dependant on another module, so if you want to enforce a dependancy to your JS code you'll have to create a module to contain/reference it.

If you're worried about having an empty module file, don't be...to formalise your library and describe it to the system properly, most likely you should be implementing the core hook_library() anyway. Then your dependant modules can simply call

drupal_add_library('mymodule', 'library');


to add your library. Putting the JS library files themselves into the sites/all/libraries folder, and referring to them from hook_library(), would probably be the best solution.