(GH-568) Prevent error when no sources defined
(GH-568) Prevent error when no sources defined

- This was tested by disabling all sources through UI
- There is a question over whether the direct usage of NugetService is correct
- This could be an underlying bug in Chocolatey.Lib

gep13/ChocolateyGUI,gep13/ChocolateyGUI,chocolatey/ChocolateyGUI,gep13/ChocolateyGUI,gep13/ChocolateyGUI,gep13/ChocolateyGUI,chocolatey/ChocolateyGUI,chocolatey/ChocolateyGUI,chocolatey/ChocolateyGUI,chocolatey/ChocolateyGUI