Do not throw error on empty LRO response

[TrucHLe]

operation.py throws an error if an LRO finishes but returns an empty response (the error message that it throws is "Unexpected state: Long-running operation had neither response nor error set"). However some APIs, such as Contact Center Insights, end LRO without setting either result or error (but set done: true). For example, please see ExportInsightsDataResponse which is an empty response that is returned after the ExportInsightsData LRO is complete.

We should update operation.py to return an empty response instead of an error when it encounters an empty LRO. For reference, we found a similar issue in the NodeJS operation library and it was fixed here.

[busunkim96]

Hi @TrucHLe,

I've opened a PR I believe will resolve the issue in #258. If you want to try it locally you can install it straight from the branch:

pip install git+https://github.com/googleapis/python-api-core.git@fix-empty-LROs

Thank you for providing a link to the NodeJS PR (and linking your sample PR)! Both were super helpful for understanding the issue. 😄