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:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

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

[ghost]

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