Fix panic during "terraform show" with empty state

It's possible to have > 1 result from the StateFilter, and not have any
instances to show.

command/state_show.go

@@ -55,6 +55,10 @@ func (c *StateShowCommand) Run(args []string) int {
 		return 1
 	}
 
+	if instance == nil {
+		return 0
+	}
+
 	is := instance.Value.(*terraform.InstanceState)
 
 	// Sort the keys

command/state_show_test.go

@@ -127,7 +127,7 @@ func TestStateShow_noState(t *testing.T) {
 }
 
 func TestStateShow_emptyState(t *testing.T) {
-	state := &terraform.State{}
+	state := terraform.NewState()
 
 	statePath := testStateFile(t, state)
 
@@ -149,6 +149,34 @@ func TestStateShow_emptyState(t *testing.T) {
 	}
 }
 
+func TestStateShow_emptyStateWithModule(t *testing.T) {
+	// empty state with empty module
+	state := terraform.NewState()
+
+	mod := &terraform.ModuleState{
+		Path: []string{"root", "mod"},
+	}
+	state.Modules = append(state.Modules, mod)
+
+	statePath := testStateFile(t, state)
+
+	p := testProvider()
+	ui := new(cli.MockUi)
+	c := &StateShowCommand{
+		Meta: Meta{
+			ContextOpts: testCtxConfig(p),
+			Ui:          ui,
+		},
+	}
+
+	args := []string{
+		"-state", statePath,
+	}
+	if code := c.Run(args); code != 0 {
+		t.Fatalf("bad: %d\n\n%s", code, ui.ErrorWriter.String())
+	}
+}
+
 const testStateShowOutput = `
 id  = bar
 bar = value