fix: set workspaceFolders to null when there is no workspace

[aminya]

Based on https://microsoft.github.io/language-server-protocol/specifications/specification-current/#initialize

    /**
     * The workspace folders configured in the client when the server starts.
     * This property is only available if the client supports workspace folders.
     * It can be `null` if the client supports workspace folders but none are
     * configured.
     *
     * @since 3.6.0
     */
    workspaceFolders?: WorkspaceFolder[] | null;

https://github.com/vuejs/vetur/issues/2561#issuecomment-743719403

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:tada: This PR is included in version 1.0.4 :tada:

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