command: fix crash on state pull with empty state

When you have no state (no local state or you just switched to a new
env) and run `terraform state pull`, it would crash.

command/state_pull.go

@@ -43,8 +43,16 @@ func (c *StatePullCommand) Run(args []string) int {
 		return 1
 	}
 
+	s := state.State()
+	if s == nil {
+		// Output on "error" so it shows up on stderr
+		c.Ui.Error("Empty state (no state)")
+
+		return 0
+	}
+
 	var buf bytes.Buffer
-	if err := terraform.WriteState(state.State(), &buf); err != nil {
+	if err := terraform.WriteState(s, &buf); err != nil {
 		c.Ui.Error(fmt.Sprintf("Failed to load state: %s", err))
 		return 1
 	}

command/state_pull_test.go

@@ -37,3 +37,27 @@ func TestStatePull(t *testing.T) {
 		t.Fatalf("expected:\n%s\n\nto include: %q", actual, expected)
 	}
 }
+
+func TestStatePull_noState(t *testing.T) {
+	tmp, cwd := testCwd(t)
+	defer testFixCwd(t, tmp, cwd)
+
+	p := testProvider()
+	ui := new(cli.MockUi)
+	c := &StatePullCommand{
+		Meta: Meta{
+			ContextOpts: testCtxConfig(p),
+			Ui:          ui,
+		},
+	}
+
+	args := []string{}
+	if code := c.Run(args); code != 0 {
+		t.Fatalf("bad: %d\n\n%s", code, ui.ErrorWriter.String())
+	}
+
+	actual := ui.OutputWriter.String()
+	if actual != "" {
+		t.Fatalf("bad: %s", actual)
+	}
+}