dev: fix govet.nilness lint issues

[alexandear]

This PR enables nilness analyzer for govet and fixes up appearing lint issues through golangci-lint repo:

❯ golangci-lint run
pkg/lint/lintersdb/manager.go:239:15: nilness: tautological condition: non-nil != nil (govet)
                if govetCfg != nil {
                            ^
pkg/lint/lintersdb/manager.go:243:18: nilness: tautological condition: non-nil != nil (govet)
                if gocriticCfg != nil {
                               ^
pkg/lint/lintersdb/manager.go:247:17: nilness: tautological condition: non-nil != nil (govet)
                if gofumptCfg != nil && gofumptCfg.LangVersion == "" {
                              ^