Fixes default value for back button when title is empty

[mateioprea]

When the current navigation stack presents a new view controller with the topbar included, if the previous view controller didn't had a {title: {text: string}} defined, the app crashed.

This commit fixes this issue which, by default, should render the back button with no text.

[yogevbd]

On which iOS version did you manage to reproduce this bug? I tried to reproduce it on iOS 13 and 11 with no luck.

[mateioprea]

@yogevbd on iOS 13.7. This is my default options config:

Navigation.setDefaultOptions({
	animations: {
		setRoot: {
			waitForRender: true,
		},
		push: {
			waitForRender: true,
		},
		pop: {
			waitForRender: true,
		},
		setStackRoot: {
			waitForRender: true,
		},
		showModal: {
			waitForRender: true,
		},
		dismissModal: {
			waitForRender: true,
		},
	},
	topBar: {
		searchBar: false,
		searchBarHiddenWhenScrolling: true,
		largeTitle: {
			visible: true,
			fontSize: 30,
			color: COLORS.DARK_BLUE_600,
			fontFamily: FONTS.ProximaNova_ExtraBold,
		},
		hideNavBarOnFocusSearchBar: false,
		backButton: {
			color: COLORS.DARK_BLUE_600,
		},
		rightButtonColor: COLORS.DARK_BLUE_600,
		title: {
			fontSize: 16,
			fontFamily: 'SFProText-Semibold',
			color: COLORS.DARK_BLUE_600,
		},
		noBorder: true,
		visible: true,
		drawBehind: true,
		animate: false,
		background: {
			color: COLORS.BACKGROUND_GRAY,
		},
		borderColor: COLORS.BACKGROUND_GRAY,
	},
});

After this is set up, when you push from a screen that doesn't have a title to another one, the app will crash.