Allow non-null outline with null stroke, fixes #18060

This is a minor fix to the chart plots to fix a corner-case use case where stroke in a theme is set to null, but outline is not.  This case can come up, particularly with area charts, in specific styling scenarios.  The fix is just a minor check where it calculates the total outline width by checking for the stroke before accessing its width property.  If it doesn't exist, it just uses 0.