Set default size for empty whiteboard

Previously, empty whiteboards were exported as a 0×0 pixel image, which
raised an error in fabric.js.
To address this issue, the default empty image is set to be a 600×600
pixel transparent png file.

Also fix #1267 that is a consequence of the aforementioned issue.

www/whiteboard/export.js

@@ -25,8 +25,9 @@ define([
                     if (c[k].y > h) { h = c[k].y + 1; }
                 });
             });
-            w = Math.min(w, MAX);
-            h = Math.min(h, MAX);
+            // Empty documents are rendered as a 600x600 transparent PNG image
+            w = w === 0 ? 600 : Math.min(w, MAX);
+            h = h === 0 ? 600 : Math.min(h, MAX);
             canvas.setWidth(w);
             canvas.setHeight(h);
             canvas.calcOffset();