core: Add missing OutputStates in synthetic state

In cases where we construct state directly rather than reading it via
the usual methods, we need to ensure that the necessary maps are
initialized correctly.

command/command_test.go

@@ -128,6 +128,7 @@ func testState() *terraform.State {
 						},
 					},
 				},
+				Outputs: map[string]*terraform.OutputState{},
 			},
 		},
 	}

terraform/context_refresh_test.go

@@ -499,6 +499,7 @@ func TestContext2Refresh_outputPartial(t *testing.T) {
 							},
 						},
 					},
+					Outputs: map[string]*OutputState{},
 				},
 			},
 		},