Allow setting UITabBarItem.title to default value

[danilobuerger]

The default value for UITabBarItem.title is nil as specified by the docs:

You should set this property before adding the item to a bar. The default value is nil.
https://developer.apple.com/documentation/uikit/uibaritem/1616412-title

This changes allows for this to be nil.

[guyca]

Hey @danilobuerger, Thanks for the PR.
Out of curiosity, Has the current logic of setting empty string cause any bugs?

[danilobuerger]

@guyca I came across this while trying to vertically center tab items on iOS when there is no title and taking the horizontal size class into account (on iPad >= iOS 11, the title is rendered to the side of the icon, not beneath, so they are already vertically centered and don't need a inset). This can't be achieved via js-land as it requires handling inside the traitCollectionDidChange callback. Anyway, while doing that, I came across this and just made the PR to get the correct setting. It doesn't actually fix a bug (that I know of).

As a side note: If centering tab items while taking horizontal size classes into account is something desired by rnn, I can submit a separate PR for that too.

[guyca]

I wasn't aware there's an issue with tab items not being centered. If you can resolve the issue in your own project, I suggest we refrain from making a PR regarding this until there are issues reported about it.