Fix bug 1236: When saving package metadata, if there is no content file, do not write empty <files> element.
Fix bug 1236: When saving package metadata, if there is no content file, do not write empty <files> element.

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branch : 1.5

dsplaisted/NuGetPackageExplorer,NuGetPackageExplorer/NuGetPackageExplorer,BreeeZe/NuGetPackageExplorer,NuGetPackageExplorer/NuGetPackageExplorer,campersau/NuGetPackageExplorer