Allow setting the StrokeDashArray of the DrawingSettings to null.

Author: dlemstra

src/Magick.NET/Settings/DrawingSettings.cs

@@ -81,6 +81,8 @@ public IEnumerable<double>? StrokeDashArray
         {
             if (value is not null)
                 _strokeDashArray = new List<double>(value).ToArray();
+            else
+                _strokeDashArray = null;
         }
     }