Remove legends if there are no edges in the graph

[PrateekKumarSingh]

Legends for now only represent the edges in the graph and if there are no edges there is no point showing legends in the graph

[PrateekKumarSingh]

Visualize legends in graph only if there are any edges:

https://github.com/PrateekKumarSingh/AzViz/blob/master/AzViz/src/private/ConvertTo-DOTLangauge.ps1#L303-L307