Exporting a graph to svg/png silently does nothing if DiagrammeRsvg isn't installed

[erikvona]

As said in the title, exporting a graph to SVG or PNG does nothing if DiagrammeRsvg is not installed.

This is caused by a minor bug in export_graph, where the code throwing an error if the package isn't installed is hidden behind a check that confirms the package is installed.

[Dennis Fu]

I experienced the same bug today, the export_graph() did nothing and threw no error. By looking into the codes I found out the cause is not installing DiagrammeRsvg and rsvg. In my case both packages are required to finally get the function working as expected.