"What would happen?" On a good day, nothing as you say.
However, it takes little to strike a tube. In dry conditions the static generated by lightly brushing the tube can do it. You can start a fitting with failed starter that way. Also a transient on the supply could easily exceed strike voltage.
If a tube strikes with no ballast it is virtually a short so the breaker trips. I imagine the glass could shatter though I've never seen that. The energy dispersed will be a function of the breaker characteristics.
