You can use the tube just fine, the center pin is just to make sure it's oriented correctly.
*BUT* - since that's now gone, you *MUST* do this manually. You can usually work it out fairly easily - have a close look at the broken tube base and you should see where the key ridge on the pin used to be. Mark the position on the side of the tube base. Now look at the socket, and you will see the same keyway - make sure the two line up. You can usually tell reasonably well if you look at the plate structures inside the glass too - if the tubes are all apparently lined up the same, that's a good sign.
Now - this is important - turn the amp on but make certain it's ON STANDBY (mute). Look round the back and make absolutely sure the filament in the broken tube has lit up. If it has, the tube is in the right way round and you can now use the amp normally - there is only one orientation the filament will work in. DO NOT flip the standby off until you've checked this - if the tube is in the wrong way, you're playing Russian Roulette with your amp when the high voltage is applied... there is one position that will work, four that won't work but will cause no harm, and three that will cause damage, out of eight. (I could be wrong about the exact odds for the second two options depending on the wiring of the tube sockets, but no more than one either way.)
With a few amps (eg some Peaveys) even this isn't safe enough because the HT is not turned off fully on standby, and you must make completely sure the tube is in the right way before turning on at all.
