Error "panic: cannot create context from nil parent" after updating to Go 1.15 or higher

After updating to Go 1.15 I'm getting this error when running my code (a unit test):

> panic: cannot create context from nil parent
>
> goroutine 14 [running]:
> testing.tRunner.func1.2(0x1211480, 0x12a3dc8) /usr/local/opt/go/libexec/src/testing/testing.go:1143
> +0x332 testing.tRunner.func1(0xc000178900) /usr/local/opt/go/libexec/src/testing/testing.go:1146 +0x4b6
> panic(0x1211480, 0x12a3dc8)
> /usr/local/opt/go/libexec/src/runtime/panic.go:965 +0x1b9
> context.WithValue(0x0, 0x0, 0x1210940, 0x12a3f58, 0x1241b80,
> 0xc00007c910, 0x12a3f58, 0xc00004a770)
> /usr/local/opt/go/libexec/src/context/context.go:521 +0x187
> /usr/local/opt/go/libexec/src/context/context.go:521 +0x187
> github.com/myrepo/pkg/test.Test_failure(0xc000765200)
> /pkg/test.go:43 +0x15f

This is my code:

ctx := context.WithValue(nil, "some string", nil)
req := http.Request{}
req = *req.WithContext(ctx)

Either use context.Background() or context.TODO() as seed if you do not have an upstream context, if you have then pass that one.

You can see here that the docs say context.Background() should be used as initial seed.
https://pkg.go.dev/context#Background

> func Background ¶
func Background() Context
Background returns a non-nil, empty Context. It is never canceled, has no values, and has no deadline. It is typically used by the main function, initialization, and tests, and as the top-level Context for incoming requests.

Generally you shouldn't have put nil there in the first place.

According to the Go 1.15 documentation passing in a nil parent is not allowed anymore:

> Creating a derived Context using a nil parent is now explicitly
> disallowed. Any attempt to do so with the WithValue, WithDeadline, or
> WithCancel functions will cause a panic.

To fix the issue I ended up using context.TODO():

ctx := context.WithValue(context.TODO(), "some string", nil)

> TODO returns a non-nil, empty Context. Code should use context.TODO
> when it's unclear which Context to use or it is not yet available
> (because the surrounding function has not yet been extended to accept
> a Context parameter).