'TypeError: Path must be a string.' error occur when revisioner.files is empty.

[loo2k]

When revisioner.files is empty, we run RevAll.manifestFile() or RevAll.versionFile() will occur the 'TypeError...' below:

path.js:8
    throw new TypeError('Path must be a string. Received ' +
    ^

TypeError: Path must be a string. Received undefined
    at assertPath (path.js:8:11)
    at Object.posix.join (path.js:479:5)
    at Revisioner.versionFile (/project/node_modules/gulp-rev-all/revisioner.js:87:24)
    at Transform._flush (/project/node_modules/gulp-rev-all/index.js:58:34)
    at Transform.<anonymous> (/project/node_modules/gulp-rev-all/node_modules/readable-stream/lib/_stream_transform.js:135:12)
    at Transform.g (events.js:260:16)
    at emitNone (events.js:67:13)
    at Transform.emit (events.js:166:7)
    at finishMaybe (/project/node_modules/gulp-rev-all/node_modules/readable-stream/lib/_stream_writable.js:371:12)
    at endWritable (/project/node_modules/gulp-rev-all/node_modules/readable-stream/lib/_stream_writable.js:378:3)

The way to fix this problem is detect whether or not revisioner.files is empty when call and RevAll.manifestFile() or RevAll.versionFile().

    RevAll.prototype.versionFile = function () {

        var revisioner = this.revisioner;

        // Drop any existing files off the stream, push the generated version file
        return Through.obj(function (file, enc, callback) {

            // Drop any existing files off the stream
            callback();

        }, function (callback) {
            if (revisioner.files.length > 0)
                this.push(revisioner.versionFile());
            callback();

        });

    };

    RevAll.prototype.manifestFile = function () {

        var revisioner = this.revisioner;

        // Drop any existing files off the stream, push the generated manifest file
        return Through.obj(function (file, enc, callback) {

            callback();

        }, function (callback) {
            if (revisioner.files.length > 0)
                this.push(revisioner.manifestFile());
            callback();

        });
    };