bf crosstrain, if qna file is empty, we should not crosstrained for qna and generated an non-empty qna file

[luhan2017]

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Describe the bug

@vishwacsena , in composer, the expected behavior is if qna file is empty, user don't need to input a qna authoring key, and the qnabuild should not be called. but we found that the crosstrain did generate a non-empty qna file.

The expected logic should be:
only of lu and qna file both exist, it trains cross the qna and lu.
if lu is empty, just keep qna, and don't do the crosstrain.
if qna is empty, just crosstrain luis, don't write anything into qna file.

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[bug]

[feich-ms]

Fixed in PR #889