Do not check for updates if libman.json file is missing

Check the libman.json file exists before checking for updates. Fixes
error logged due to a null filename being used for the libman.json
file when checking for updates.

Could not get Library Manager updates for ''
System.ArgumentNullException: Value cannot be null. (Parameter 'key')
   at System.Collections.Generic.Dictionary`2.FindValue(TKey key)
   at System.Collections.Generic.Dictionary`2.ContainsKey(TKey key)
   at Microsoft.Web.LibraryManager.Vsix.Contracts.DependenciesFactory.FromConfigFile(String configFilePath)
   at MonoDevelop.LibraryManager.Updates.UpdatedLibrariesService.GetUpdatesAsync(FilePath configFilePath, CancellationToken cancellationToken)
   at MonoDevelop.LibraryManager.Updates.UpdatedLibrariesService.SafeGetUpdatesAsync(FilePath configFilePath, CancellationToken cancellationToken)

src/MonoDevelop.LibraryManager/Updates/UpdatedLibrariesService.cs

@@ -83,6 +83,10 @@ async Task CheckForUpdatesInternalAsync(Solution solution, CancellationToken can
                 }
 
                 FilePath manifestFileName = project.GetManifestFileName();
+                if (manifestFileName.IsNull)
+                {
+                    continue;
+                }
 
                 List<UpdatedLibraryInfo> manifestUpdates = await SafeGetUpdatesAsync(
                     manifestFileName,