Emit metric tracking empty responses from prometheus

[drolando-stripe]

We'd like to have a way to monitor the number of Keda errors due to empty responses from prometheus after enabling the ignoreNullValues flag for most of our prometheus triggers.

Right now this error gets logged but the error metric that Keda emits is generic and doesn't differentiate by error type.

Checklist

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Fixes #7062

[github-actions]

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