Refactor code to use backend config state's SetConfig method, protect against nil receiver

[SarahFrench]

For context, the SetConfig method on BackendConfigState is only used in the context of tests currently. Whenever a BackendConfigState is assembled the config is set directly by assigning a value to ConfigRaw:

terraform/internal/command/meta_backend.go

Lines 1055 to 1064 in a9b67a6

	// Store the metadata in our saved state location
	s := sMgr.State()
	if s == nil {
	s = workdir.NewBackendStateFile()
	}
	s.Backend = &workdir.BackendConfigState{
	Type: c.Type,
	ConfigRaw: json.RawMessage(configJSON),
	Hash: uint64(cHash),
	}

That configJSON value is constructed in the code immediately preceding the above snippet:

terraform/internal/command/meta_backend.go

Lines 1049 to 1053 in a9b67a6

	configJSON, err := ctyjson.Marshal(configVal, b.ConfigSchema().ImpliedType())
	if err != nil {
	diags = diags.Append(fmt.Errorf("Can't serialize backend configuration as JSON: %s", err))
	return nil, diags
	}

This PR refactors that code to instead use the SetConfig method. This method includes the logic for marshalling cty data to JSON, so replaces the equivalent code above that gets the implied type of a schema and marshals the cty value to json.

Also, this PR adds a nil receiver check to the method.

Target Release

1.13.x

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.