(maint) Fix node graph generation
(maint) Fix node graph generation

Previously the langauge server would optionally return a property called
dotContent when an error occured during Node Graph generation.  However the
language client always assumed that dotContent was returned.  This commit
updates the client to use an empty string if dotContent is not returned by the
server.

jpogran/puppet-vscode,jpogran/puppet-vscode,glennsarti/puppet-vscode