0.7.4

• fix bug: a sheet with neither an index nor a fixed row/col renders every track
  collapsed, since regrid emits repeat(0, max-content) - repeat() takes an
  integer of at least 1, so the whole grid-template-* declaration is dropped.
• fix bug: an all empty row collapses, since an empty cell has no line box and
  .cell had no min-height to fall back on. only visible without an index column,
  whose row number used to be what kept such rows open.
• add a no index / no fixed example to the demo page.