loadContents function will fail when empty and non-empty lu or qna files list are provided #924

Versions

bf-cli master branch
Composer reported issue since they have empty lu or qna files in many scenarios.

Describe the bug

When users call loadContents function in builder.ts of lubuild or qnabuild, it will not load files and create dialogs assets correctly when parsing empty files. The root cause it that luMerger will throw exception if we don't filter those empty files. The fix is straightforward, just filter empty files before calling luMerger and avoid creating recognizers for those empty files.

To Reproduce

Steps to reproduce the behavior:

1. use luis build or qnamaker build cli
2. use one non-empty and one empty files as input
3. see the build results, you will find the non-empty file is not published

Expected behavior

loadContents can handle empty files correctly.

Screenshots

If applicable, add screenshots to help explain your problem.

Additional context

Add any other context about the problem here.

[bug]