Check call.trailing_metadata() for None before iterating it

[igorzhilianin]

In case of handling exception within intercepted call, grpc._interceptor._FailureOutcome.trailing_metadata() will return None, causing
rpc_status.from_call() called from inside of interceptor to raise "TypeError: 'NoneType' object is not iterable".

There is a fix.

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[thelinuxfoundation]

Thank you for your pull request. Before we can look at your contribution, we need to ensure all contributors are covered by a Contributor License Agreement.

After the following items are addressed, please respond with a new comment here, and the automated system will re-verify.

• User @igorzhilianin isn't covered by a CLA. They will need to complete the form at https://identity.linuxfoundation.org/projects/cncf

Regards,
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[lidizheng]

Sorry for the late review. I thought one LGTM is sufficient.