function getVersionIdResHeader(verCfg, objectMD) {
    if (verCfg) {
        if (objectMD.isNull || !objectMD.versionId) {
            return 'null';
        }
        return versionIdUtils.encode(objectMD.versionId);
    }
    return undefined;
}
getVersionIdResHeader - return encrypted version ID if appropriate
@param {object} [verCfg] - bucket versioning configuration
@param {object} objectMD - object metadata
@return {(string|undefined)} - undefined or encrypted version ID
(if not 'null')