fix(layer-control): keep line in legend icon when merge-group fill is invisible

[PZahnen]

Reported for a boundary style: the legend icons of its merge-groups stayed empty.

• Cause: when a merge-group contains a fill layer, its line layers are left out of the legend icon. The reported style combines each boundary line with a fully transparent fill (fill-opacity: 0). Only that invisible fill was left in the icon. The order of the layers makes no difference.
• Fix: fills with a constant fill-opacity: 0 no longer count for this rule, so the icon shows the line. Merge-groups with a visible fill are unchanged.

New Storybook story LegendIcons / Invisible Fill covers both cases.