An LSP initialize whose params include a workspaceFolders array could throw a NullReferenceException inside PsesLanguageServer.OnInitialize, leaving the handshake incomplete and the server unusable for clients that send workspaceFolders.

The handler added every folder verbatim and later dereferenced WorkspaceFolder.Uri (in OnInitialize and in WorkspaceService.GetRelativePath/FindFileInWorkspace/WorkspacePaths). An absent/uninitialized collection or a folder entry without a Uri therefore produced an NRE.

Changes

• PsesLanguageServer.GetValidWorkspaceFolders (new internal helper): treats an absent collection or any folder lacking a Uri as "no folder", filtering them out before AddRange so nothing with a null Uri reaches the workspace.
• OnInitialize: feeds workspace folders through the helper instead of adding them directly.
• InitialWorkingDirectory fallback: null-guards the Uri dereference (FirstOrDefault()?.Uri?.GetFileSystemPath()).
• Tests: cover null collection, empty collection, and a mix of null folder / null-Uri folder / valid folder.

internal static IEnumerable<WorkspaceFolder> GetValidWorkspaceFolders(IEnumerable<WorkspaceFolder> workspaceFolders)
    => workspaceFolders?.Where(static folder => folder?.Uri is not null)
        ?? Enumerable.Empty<WorkspaceFolder>();