[SPARK-15096][ML] LogisticRegression MultiClassSummarizer numClasses can fail...

[SPARK-15096][ML] LogisticRegression MultiClassSummarizer numClasses can fail if no valid labels are found

## What changes were proposed in this pull request?

(Please fill in changes proposed in this fix)
Throw better exception when numClasses is empty and empty.max is thrown.

## How was this patch tested?

(Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests)
Add a new unit test, which calls histogram with empty numClasses.

Author: wm624@hotmail.com <wm624@hotmail.com>

Closes #12969 from wangmiao1981/logisticR.