Revert #7464 and take allow an empty state

The error for a nil remote state was showing up in other places. While
we could check for the error on all calls to Refresh(), this restores
the previous behavior, while preventing possible nil values in the
state.

The effect here is the same, because rather than the RefreshState
switching on

     cached != nil && durable == nil

it will switch on

    durable.Serial < cached.Serial

which sets the same value to RefreshResult().

command/state.go

@@ -231,6 +231,7 @@ func remoteState(
 
 	if refresh {
 		// Refresh the cache
+		// It's OK if we didn't find a remote state.
 		if err := cache.RefreshState(); err != nil {
 			return nil, errwrap.Wrapf(
 				"Error reloading remote state: {{err}}", err)

state/remote/remote_test.go

@@ -46,8 +46,8 @@ func TestRemoteClient_noPayload(t *testing.T) {
 	s := &State{
 		Client: nilClient{},
 	}
-	if err := s.RefreshState(); err != ErrRemoteStateNotFound {
-		t.Fatal("expected ErrRemoteStateNotFound, got", err)
+	if err := s.RefreshState(); err != nil {
+		t.Fatal("error refreshing empty remote state")
 	}
 }
 

state/remote/state.go

@@ -2,13 +2,10 @@ package remote
 
 import (
 	"bytes"
-	"errors"
 
 	"github.com/hashicorp/terraform/terraform"
 )
 
-var ErrRemoteStateNotFound = errors.New("no remote state found")
-
 // State implements the State interfaces in the state package to handle
 // reading and writing the remote state. This State on its own does no
 // local caching so every persist will go to the remote storage and local
@@ -38,7 +35,14 @@ func (s *State) RefreshState() error {
 	}
 
 	if payload == nil {
-		return ErrRemoteStateNotFound
+		// the response was empty, so treat it as an empty state
+		emptyState := terraform.NewState()
+		// this needs to be superceded by any other serial, incluing 0
+		emptyState.Serial = -1
+
+		s.state = emptyState
+		s.readState = emptyState
+		return nil
 	}
 
 	state, err := terraform.ReadState(bytes.NewReader(payload.Data))