Simplify the logic in PerfCompare.js

[llxia]

In PerfCompare.js, we are using undefined, null, "" to represent different meanings. I think we should simplify the logic.

• [ ] higherbetter should have only two state. i.e., true or false. There is no need to have a third state undefined. The default value for higherbetter in BenchmarkMetric.js should be true. It only needs to set explicitly if it is false.

• [ ] Remove sepcific checks for undefined, null, "". Update code as the following:

        if (resBenchmarkRunsJson === undefined || (Object.keys(resBenchmarkRunsJson).length === 0 && resBenchmarkRunsJson.constructor === Object) ||
                resBenchmarkRunsJson.baselineCSV === undefined || resBenchmarkRunsJson.testCSV === undefined) {

to

if (!resBenchmarkRunsJson || !resBenchmarkRunsJson.baselineCSV || !resBenchmarkRunsJson.testCSV)

Please check the code and update similar checks.