Make StaffSymbol.line-positions always set

Before, line-positions was only for user overrides. All places accessing it needed to fall back on using line-count if it was empty. Now, the default for line-positions is a callback that computes the usual line positions from line-count, obviating those fallbacks.

This entails a minor change: before, \override Staff.StaffSymbol.line-positions = #'() would specify to revert to the behavior of computing staff lines from line-count, but now it means to print no staff line, like line-count = 0 (which makes more sense). convert-ly takes care of this.