Handle when there is no push nuget feed url specified for pushing the solution nuget package.

[dazinator]

If you specify /p:CreateSolutionDeploymentPackage but don't also specify the url to push that package to, the build fails - as it goes on to try and push to a blank url.

To replicate:

msbuild "G:\test\test.sln" /p:CreateSolutionDeploymentPackage=true /p:
NuGetExeFilePath="packages\NuGet.CommandLine.2.8.2\tools\NuGet.exe" /p:BuildVers
ionNumber=12345

The error:

Build FAILED.

"G:\test\test.sln" (default target) (1) ->
(CreateAndPushNugetPackage target) ->
  G:\test\after.test.sln.targets(41,5): error MSB3073: The command ""packages\NuGet.Comm
andLine.2.8.2\tools\NuGet.exe" push G:\test\\*.nupkg  -s  " exited with code 1. [G:\test\test.sln]