function _prepareNullVersionDeletion(bucketName, objKey, options, mst, log, cb) {
    const nullOptions = {};
    if (!options.deleteData) {
        return process.nextTick(cb, null, nullOptions);
    }
    if (options.versionId === mst.versionId) {
        // no need to get another key as the master is the target
        nullOptions.dataToDelete = mst.objLocation;
        return process.nextTick(cb, null, nullOptions);
    }
    if (options.versionId === 'null') {
        // deletion of the null key will be done by the main metadata
        // PUT via this option
        nullOptions.deleteNullKey = true;
    }
    return metadata.getObjectMD(bucketName, objKey, options, log,
        (err, versionMD) => {
            if (err) {
                // the null key may not exist, hence it's a normal
                // situation to have a NoSuchKey error, in which case
                // there is nothing to delete
                if (err.is.NoSuchKey) {
                    log.debug('null version does not exist', {
                        method: '_prepareNullVersionDeletion',
                    });
                } else {
                    log.warn('could not get null version metadata', {
                        error: err,
                        method: '_prepareNullVersionDeletion',
                    });
                }
                return cb(err);
            }
            if (versionMD.location) {
                const dataToDelete = Array.isArray(versionMD.location) ?
                      versionMD.location : [versionMD.location];
                nullOptions.dataToDelete = dataToDelete;
            }
            return cb(null, nullOptions);
        });
}
check existence and get location of null version data for deletion
@param {string} bucketName - name of bucket
@param {string} objKey - name of object key
@param {object} options - metadata options for getting object MD
@param {string} options.versionId - version to get from metadata
@param {object} mst - info about the master version
@param {string} mst.versionId - the master version's version id
@param {RequestLogger} log - logger instanceof
@param {function} cb - callback
@return {undefined} - and call callback with (err, dataToDelete)