Check if current path is null for path construction operators

src/UglyToad.PdfPig/Graphics/Operations/PathConstruction/AppendStraightLineSegment.cs

@@ -41,6 +41,8 @@
         /// <inheritdoc />
         public void Run(IOperationContext operationContext)
         {
+            if (operationContext.CurrentSubpath == null) return;
+
             var endPoint = operationContext.CurrentTransformationMatrix.Transform(new PdfPoint(X, Y));
             operationContext.CurrentSubpath.LineTo(endPoint.X, endPoint.Y);
             operationContext.CurrentPosition = endPoint;