Workaround for emptying FSharp.Editor cache due to duplicate ProjectSystem events

[vzarytovskii]

Workaround, until we have a proper fix from the project system or move to Roslyn as a source of truth:
Do not update cache if we receive completely empty list from project system.
A workaround for #12982.
More details:

When we open the solution, HandleCommandLineChanges is getting called by project system (CPS):

fsharp/vsintegration/src/FSharp.Editor/LanguageService/LanguageService.fs

Lines 347 to 365 in 12c8309

	member _.HandleCommandLineChanges(path:string, sources:ImmutableArray<CommandLineSourceFile>, _references:ImmutableArray<CommandLineReference>, options:ImmutableArray<string>) =
	use _logBlock = Logger.LogBlock(LogEditorFunctionId.LanguageService_HandleCommandLineArgs)
	let projectId =
	match Microsoft.CodeAnalysis.ExternalAccess.FSharp.LanguageServices.FSharpVisualStudioWorkspaceExtensions.TryGetProjectIdByBinPath(workspace, path) with
	| true, projectId -> projectId
	| false, _ -> Microsoft.CodeAnalysis.ExternalAccess.FSharp.LanguageServices.FSharpVisualStudioWorkspaceExtensions.GetOrCreateProjectIdForPath(workspace, path, projectDisplayNameOf path)
	let path = Microsoft.CodeAnalysis.ExternalAccess.FSharp.LanguageServices.FSharpVisualStudioWorkspaceExtensions.GetProjectFilePath(workspace, projectId)
	let getFullPath p =
	let p' =
	if Path.IsPathRooted(p) || path = null then p
	else Path.Combine(Path.GetDirectoryName(path), p)
	Path.GetFullPathSafe(p')
	let sourcePaths = sources |> Seq.map(fun s -> getFullPath s.Path) |> Seq.toArray
	let workspaceService = workspace.Services.GetRequiredService<IFSharpWorkspaceService>()
	workspaceService.FSharpProjectOptionsManager.SetCommandLineOptions(projectId, sourcePaths, options)

When project system calls us, it sends us project id, source files and options, and we store them in cache:

fsharp/vsintegration/src/FSharp.Editor/LanguageService/FSharpProjectOptionsManager.fs

Lines 540 to 541 in 12c8309

	member this.SetCommandLineOptions(projectId, sourcePaths, options: ImmutableArray<string>) =
	reactor.SetCommandLineOptions(projectId, sourcePaths, options.ToArray())

Project system calls us here:
https://github.com/dotnet/project-system/blob/2d8184bf78a11568ae502597ed5ba6d24bdd736c/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/LanguageServices/FSharp/FSharpCommandLineParserService.cs#L83-L94

Notification, apparently, originates from here:

https://github.com/dotnet/project-system/blob/9815fdcba096f21ae64a0dd1803d61b21b0c8b62/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/LanguageServices/Handlers/CommandLineNotificationHandler.cs#L26-L33

We use that cache afterwards, to get documents from it and then apply colouring, tooltips, etc.

However, what's happening now is:

1. We open a solution
2. We receive project system"events" once for every project, we store it in cache, and we successfully store everything in cache.
3. We close solution, we empty the cache.
4. We REopen the solution, we receive project system "events" for every projects, we store it in cache, BUT
5. We again receive project system events, for each project (usually, for all of them, except if there are any documents open already).
   HOWEVER those events contain empty source file list as well as options list, and we are rewriting the cache with empty lists.
6. Later on, when we use this cache, we don't find any documents in it, and cannot apply colour, tooltips, etc.

[vzarytovskii]

Converting to draft, since I will need to do some extensive manual testing before merging.

[KevinRansom]

Undrafting.

[vzarytovskii]

OK, I have verified that it works (there's still some lag between opening the document and colouring kick in, but it's most likely due to my slow laptop).