allow setting -backend-config='' to unset override

There is currently no way to unset -backend-config during init, since
not setting that option assumes the user will use the saved config.
Allow setting `-backend-config=""` to specify no overrides.

command/init.go

@@ -299,7 +299,7 @@ func (c *InitCommand) Run(args []string) int {
 
 	if back == nil {
 		// If we didn't initialize a backend then we'll try to at least
-		// instantiate one. This might fail if it wasn't already initalized
+		// instantiate one. This might fail if it wasn't already initialized
 		// by a previous run, so we must still expect that "back" may be nil
 		// in code that follows.
 		var backDiags tfdiags.Diagnostics
@@ -675,6 +675,12 @@ func (c *InitCommand) backendConfigOverrideBody(flags rawFlags, schema *configsc
 		synthVals = make(map[string]cty.Value)
 	}
 
+	if len(items) == 1 && items[0].Value == "" {
+		// Explicitly remove all -backend-config options.
+		// We do this by setting an empty but non-nil ConfigOverrides.
+		return configs.SynthBody("-backend-config=''", synthVals), diags
+	}
+
 	for _, item := range items {
 		eq := strings.Index(item.Value, "=")
 

command/init_test.go

@@ -454,6 +454,22 @@ func TestInit_backendConfigKVReInit(t *testing.T) {
 	if cfg["path"] != "test" {
 		t.Fatalf(`expected backend path="test", got path="%v"`, cfg["path"])
 	}
+
+	// override the -backend-config options by settings
+	args = []string{"-input=false", "-backend-config", ""}
+	if code := c.Run(args); code != 0 {
+		t.Fatalf("bad: \n%s", ui.ErrorWriter.String())
+	}
+
+	// make sure the backend is configured how we expect
+	configState = testDataStateRead(t, filepath.Join(DefaultDataDir, DefaultStateFilename))
+	cfg = map[string]interface{}{}
+	if err := json.Unmarshal(configState.Backend.ConfigRaw, &cfg); err != nil {
+		t.Fatal(err)
+	}
+	if cfg["path"] != nil {
+		t.Fatalf(`expected backend path="<nil>", got path="%v"`, cfg["path"])
+	}
 }
 
 func TestInit_backendConfigKVReInitWithConfigDiff(t *testing.T) {