Run nilaway to detect potential nil errors in codebase

[chmouel]

https://github.com/uber-go/nilaway

NilAway is a static analysis tool that seeks to help developers avoid nil
panics in production by catching them at compile time rather than runtime.
NilAway is similar to the standard nilness analyzer, however, it employs much
more sophisticated and powerful static analysis techniques to track nil flows
within a package as well across packages, and report errors providing users
with the nilness flows for easier debugging.

it's not included in golangci-lint so we have to run it manually.

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