[release/8.0-staging] Return null instead of throwing in ServiceDescriptor.ImplementationInstance\Type\Factory if a keyed service

[steveharter]

Backport of #105776 to release/8.0-staging

Customer Impact

• Customer reported
• Found internally

See issue #95789 and duplicate issue #99622. In 8.0, when supported for keyed services was added, 3 existing properties were changed to throw InvalidOperationException in cases where they were not expected to be called. However, this caused some backwards compatibility issues for various scenarios including tooling.

Regression

• Yes
• No

The 3 affected properties before the keyed services feature was added in 8.0 did not throw; once the keyed services feature was added they can throw, by design, in cases where they were not expected to be called.

Testing

The tests verify the expected behavior of returning null instead of throwing; existing tests were modified in 9.0 for the new behavior.

Risk

Low, this changes 3 properties that were changed in 8.0 to throw an InvalidOperationException in unexpected cases; with this PR they return null instead.

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