Automatic merge from submit-queue (batch tested with PRs 39694, 39383, 39651, 39691, 39497)

HPA Controller: Check for 0-sum request value

In certain conditions in which the set of metrics returned by Heapster
is completely disjoint from the set of pods returned by the API server,
we can have a request sum of zero, which can cause a panic (due to
division by zero).  This checks for that condition.

Fixes #39680

**Release note**:

```release-note
Fixes an HPA-related panic due to division-by-zero.
```