chore(testing): do not emit nil error formatting

[grayside]

Description

When using the cloudrunci retry mechanism, if multiple retries are made and the test fails without an error being reported, a nil error comes out. This was a distraction.

request: no acceptable response after 5 retries: %!w(<nil>))

This PR removes the nil error from output.

Checklist

• I have followed Contributing Guidelines from CONTRIBUTING.MD
• Tests pass: go test -v ./.. (see Testing)
• Code formatted: gofmt (see Formatting)
• Vetting pass: go vet (see Formatting)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved