Enable "nilnil" linter in golangci-lint

[tuminoid]

Enable "nilnil" linter in golangci-lint. Fix any warnings enabling it causes.

/area ci
/help
/triage accepted

[metal3-io-bot]

@tuminoid: The label(s) area/ci cannot be applied, because the repository doesn't have them.

In response to this:

Enable "nilnil" linter in golangci-lint. Fix any warnings enabling it causes.

/area ci
/help
/triage accepted

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[andriisoldatenko]

this one a bit tricky, it's hard to fix it without project experience :(

I only can enable linter and add //nolint:nilnil or so on those lines

INFO [runner] linters took 7.612566458s with stages: goanalysis_metalinter: 7.597061583s
controllers/metal3.io/baremetalhost_controller.go:713:3: return both a `nil` error and an invalid value: use a sentinel error instead (nilnil)
                return nil, nil
                ^
controllers/metal3.io/baremetalhost_controller.go:2040:4: return both a `nil` error and an invalid value: use a sentinel error instead (nilnil)
                        return nil, nil
                        ^
controllers/metal3.io/baremetalhost_controller.go:2143:3: return both a `nil` error and an invalid value: use a sentinel error instead (nilnil)
                return nil, nil
                ^
controllers/metal3.io/baremetalhost_controller.go:2156:3: return both a `nil` error and an invalid value: use a sentinel error instead (nilnil)
                return nil, nil
                ^
pkg/provisioner/ironic/ironic.go:283:2: return both a `nil` error and an invalid value: use a sentinel error instead (nilnil)
        return nil, nil

[tuminoid]

Thanks for checking it out @andriisoldatenko. It is a good finding that it isn't suitable for external contributors!

I'll remove the help-wanted in this case, and let's see if a maintainer picks it up.

/remove-help