opentelemetry-test-utils: don't crash in TestBase.get_sorted_metrics without metrics

[xrmx]

Description

When calling the functions with no metrics recorded it will crash because self.memory_metrics_reader.get_metrics_data() is None

    def get_sorted_metrics(self):
        resource_metrics = (
>           self.memory_metrics_reader.get_metrics_data().resource_metrics
        )
E       AttributeError: 'NoneType' object has no attribute 'resource_metrics'

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• tox

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated