[SPARK-4256] Make Binary Evaluation Metrics functions defined in cases where there ar...

...e 0 positive or 0 negative examples.
Scala style (braces):
```
override def apply(c: BinaryConfusionMatrix): Double = {
  if (c.numNegatives == 0) {
    0.0
  } else {
    c.numFalsePositives.toDouble / c.numNegatives
  }
}
```