Implement golang static analyzer findings

[mfielding]

Running a few golang static analyzers on the code under /v2, they have a few recommendations:

1. Unnecessary nil checks that are always true (https://staticcheck.io/docs/checks#SA4031, https://pkg.go.dev/golang.org/x/tools/go/analysis/passes/nilness)
2. Unkeyed composite literals in database/sql.Null types (https://pkg.go.dev/golang.org/x/tools/go/analysis/passes/composite)
3. Using strings.EqualFold to compare string equality, which is much faster (https://staticcheck.dev/docs/checks#SA6005)
4. Two cases in custom_types.go where we don't check for errors (https://staticcheck.io/docs/checks#SA4006)
5. Adding spaces after // comments for readability