[SPARK-19400][ML] Allow GLM to handle intercept only model

## What changes were proposed in this pull request?
Intercept-only GLM is failing for non-Gaussian family because of reducing an empty array in IWLS. The following code `val maxTolOfCoefficients = oldCoefficients.toArray.reduce { (x, y) => math.max(math.abs(x), math.abs(y))` fails in the intercept-only model because `oldCoefficients` is empty. This PR fixes this issue.

yanboliang srowen imatiach-msft zhengruifeng

## How was this patch tested?
New test for intercept only model.

Author: actuaryzhang <actuaryzhang10@gmail.com>

Closes #16740 from actuaryzhang/interceptOnly.