As is documented here among other places, a couple of the interceptors sent after United Flight 93 on September 11, 2001 were scrambled so hastily they did not have time for the ordies to visit and arm them up. While not having live missiles on you at all times makes sense for a variety of reasons -- I can't understand why a fighter jet would be flying around with 0 rounds of ammo for their gun, considering how much of a W&B impact the ammo load has.
Furthermore, even if the media coverage was referring to them being loaded with training slugs instead of live ammunition -- you can seriously damage an airplane with 20mm slugs, as one Grumman test pilot (among other folks, it seems) found out the very hard way! Sure, you'd have to use quite a bit of cannon fire to do it, but it'd still be preferable to a last-ditch ramming attack, no?
So, how could a fighter be scrambled into a situation where their only option was to ram their target?