Draft: ShapeString Fuse caused single face string to lose fill

The `faces.concatenate` function can return a Face or a Compound. The code did not take that into account.

src/Mod/Draft/draftobjects/shapestring.py

@@ -159,6 +159,10 @@ class ShapeString(DraftObject):
                 if fill and obj.Fuse:
                     ss_shape = shapes[0].fuse(shapes[1:])
                     ss_shape = faces.concatenate(ss_shape)
+                    # Concatenate returns a Face or a Compound. We always
+                    # need a Compound as we use ss_shape.SubShapes later.
+                    if ss_shape.ShapeType == "Face":
+                        ss_shape = Part.Compound([ss_shape])
                 else:
                     ss_shape = Part.Compound(shapes)
                 cap_char = Part.makeWireString("M", obj.FontFile, obj.Size, obj.Tracking)[0]