toSecret' "return nil, nil" is error-prone

[machine424]

here

common/config/http_config.go

Lines 781 to 797 in 098669c

	func toSecret(secretManager SecretManager, text Secret, file, ref string) (SecretReader, error) {
	if text != "" {
	return NewInlineSecret(string(text)), nil
	}
	if file != "" {
	return NewFileSecret(file), nil
	}
	if ref != "" {
	if secretManager == nil {
	return nil, errors.New("cannot use secret ref without manager")
	}
	return &refSecret{
	ref: ref,
	manager: secretManager,
	}, nil
	}
	return nil, nil

because the error is nil, we may/would be moving nils around, leading to nil pointer dereference

may be the cause of prometheus/prometheus#16622

[alliasgher]

Opened #897 for this.