fix(stackdriver): handle null timeSeries and empty points (backport #1047)

[mergify]

Adding an if statement to check if timeSeries is null and if the points are empty + tests

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This is an automatic backport of pull request #1047 done by Mergify.

[mergify]

Cherry-pick of 6684f9d has failed:

On branch mergify/bp/release-1.28.x/pr-1047
Your branch is up to date with 'origin/release-1.28.x'.

You are currently cherry-picking commit 6684f9dd.
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  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   kayenta-stackdriver/src/main/java/com/netflix/kayenta/stackdriver/metrics/StackdriverMetricsService.java
	deleted by us:   kayenta-stackdriver/src/test/java/com/netflix/kayenta/stackdriver/metrics/StackdriverMetricsServiceTest.java

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