Handle passing null to cpp instance manager

[a-maurice]

Description

Provide details of the change, and generalize the change in the PR title above.

The internal GetInstance classes are sometimes expected to return null, for example Storage does if passing an invalid bucket. Instead of checking for null in each case, it seems better to let the cpp_instance_manager handle when null is passed in, especially instead of storing those nulls, as it currently does, assuming assertions are off.

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Testing

Describe how you've tested these changes.

Building and running locally

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Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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[jonsimantov]

Do we check the result anywhere?

[a-maurice]

Do we check the result anywhere?

Only place I saw was in the test file (that is currently not used).