Compatibility helper to get workspace folders. Checks these members: 1) If workspaceFolders has any entries, it is returned as-is. 2) Otherwise, if rootUri is non-empty, it is returned as the only workspace, with fallbackRootName as name. 3) Otherwise, if rootPath is non-empty, it is converted to a URI and returned as the only workspace, with fallbackRootName as name. 4) Otherwise, empty array [] is returned.