Hide marker line for blank points in scatter plots

- Prevent marker line borders from rendering on blank data points by setting line width to 0 when `d.isBlank` is true in the `singlePointStyle()` function
- Add `dash` and `dashsrc` schema attributes for marker line styling across multiple chart types to support customisable dash patterns
- Ensure blank points appear correctly without visible outlines while maintaining proper styling for non-blank markers

Author: chrimaho

src/components/drawing/index.js

@@ -1060,7 +1060,7 @@ drawing.singlePointStyle = function (d, sel, trace, fns, gd, pt) {
 
         if (lineWidth) {
             Color.stroke(sel, lineColor);
-            drawing.dashLine(sel, lineDash, lineWidth);
+            drawing.dashLine(sel, lineDash, d.isBlank ? 0 : lineWidth);
         }
     }
 };