added null checks for aggregate stats
added null checks for aggregate stats

KuduApps/NuGetGallery,KuduApps/NuGetGallery,skbkontur/NuGetGallery,mtian/SiteExtensionGallery,grenade/NuGetGallery_download-count-patch,grenade/NuGetGallery_download-count-patch,skbkontur/NuGetGallery,mtian/SiteExtensionGallery,JetBrains/ReSharperGallery,JetBrains/ReSharperGallery,skbkontur/NuGetGallery,KuduApps/NuGetGallery,projectkudu/SiteExtensionGallery,ScottShingler/NuGetGallery,ScottShingler/NuGetGallery,projectkudu/SiteExtensionGallery,JetBrains/ReSharperGallery,grenade/NuGetGallery_download-count-patch,KuduApps/NuGetGallery,projectkudu/SiteExtensionGallery,ScottShingler/NuGetGallery,mtian/SiteExtensionGallery,KuduApps/NuGetGallery