command/init: initialize backend even if not set in the config

We need to initialize the backend even if the config has no backend set.
This allows `init` to work when unsetting a previously set backend.
Without this, there was no way to unset a backend.

command/init.go

@@ -125,13 +125,17 @@ func (c *InitCommand) Run(args []string) int {
 		}
 
 		// If we're requesting backend configuration and configure it
-		hasBackend := conf.Terraform != nil && conf.Terraform.Backend != nil
-		if flagBackend && hasBackend {
+		if flagBackend {
 			header = true
 
+			// Only output that we're initializing a backend if we have
+			// something in the config. We can be UNSETTING a backend as well
+			// in which case we choose not to show this.
+			if conf.Terraform != nil && conf.Terraform.Backend != nil {
 				c.Ui.Output(c.Colorize().Color(fmt.Sprintf(
 					"[reset][bold]" +
 						"Initializing the backend...")))
+			}
 
 			opts := &BackendOpts{
 				ConfigPath: path,

command/init_test.go

@@ -1,6 +1,7 @@
 package command
 
 import (
+	"io/ioutil"
 	"os"
 	"path/filepath"
 	"strings"
@@ -236,6 +237,63 @@ func TestInit_backend(t *testing.T) {
 	}
 }
 
+func TestInit_backendUnset(t *testing.T) {
+	// Create a temporary working directory that is empty
+	td := tempDir(t)
+	copy.CopyDir(testFixturePath("init-backend"), td)
+	defer os.RemoveAll(td)
+	defer testChdir(t, td)()
+
+	{
+		ui := new(cli.MockUi)
+		c := &InitCommand{
+			Meta: Meta{
+				ContextOpts: testCtxConfig(testProvider()),
+				Ui:          ui,
+			},
+		}
+
+		// Init
+		args := []string{}
+		if code := c.Run(args); code != 0 {
+			t.Fatalf("bad: \n%s", ui.ErrorWriter.String())
+		}
+
+		if _, err := os.Stat(filepath.Join(DefaultDataDir, DefaultStateFilename)); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+	}
+
+	{
+		// Unset
+		if err := ioutil.WriteFile("main.tf", []byte(""), 0644); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+
+		// Run it again
+		defer testInteractiveInput(t, []string{"yes", "yes"})()
+
+		ui := new(cli.MockUi)
+		c := &InitCommand{
+			Meta: Meta{
+				ContextOpts: testCtxConfig(testProvider()),
+				Ui:          ui,
+			},
+		}
+
+		args := []string{}
+		if code := c.Run(args); code != 0 {
+			t.Fatalf("bad: \n%s", ui.ErrorWriter.String())
+		}
+
+		s := testStateRead(t, filepath.Join(
+			DefaultDataDir, DefaultStateFilename))
+		if !s.Backend.Empty() {
+			t.Fatal("should not have backend config")
+		}
+	}
+}
+
 func TestInit_backendConfigFile(t *testing.T) {
 	// Create a temporary working directory that is empty
 	td := tempDir(t)