mark nil errors as nil instead of non-retryable

[modular-magician]

This should fix the Retry Transport: Stopping retries, last request failed with non-retryable error: empty non-retryable error received. This is a bug with the Terraform provider and should be reported as a GitHub issue in the provider repository. messages in the logs that we've been seeing lately.

If this PR is for Terraform, I acknowledge that I have:

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Release Note Template for Downstream PRs (will be copied)

all: fixed misleading "empty non-retryable error" message that was appearing in debug logs

Derived from GoogleCloudPlatform/magic-modules#4124