fix: Do not disable fallback if the metric type is not explicitly configured

[salvacorts]

The fallback mechanism should only work if the metric type is AverageValue, which is the default (according to the docs).

Problem is, if you do not explicitly declare the metric type, KEDA prints the following error and disables the fallback mechanism:

2024-08-02T15:01:04Z ERROR scaledobject-validation-webhook validation error {"name": "foo", "error": "MetricType=, but Fallback can only be enabled for triggers with metric of type AverageValue"}

This PR fixes this by checking if the metric type is empty, in which case it's evaluated as AverageValue.

Checklist

• When introducing a new scaler, I agree with the scaling governance policy
• I have verified that my change is according to the deprecations & breaking changes policy
• Tests have been added
• Changelog has been updated and is aligned with our changelog requirements
• A PR is opened to update our Helm chart (repo) (if applicable, ie. when deployment manifests are modified)
• A PR is opened to update the documentation on (repo) (if applicable)
• Commits are signed with Developer Certificate of Origin (DCO - learn more)

Fixes #

Relates to #

[salvacorts]

I just noticed this one is already open #5962 and addressing something similar. So we should address this there.

I'll close this one but briefly reviewing the already opened one, I find a function that get the metric or returns the configured would be handy.

Closing!