fix: avoid stringified "null" and "undefined" values

[jaulz]

No description provided.

[rszalski]

Would love to see this merged in. One case when this is problematic is when you defined optional params in your navigators, e.g.

Home: {
  screen: HomeScreen,
  path: 'link/:requiredParam/:optionalParam?'
},

Then you need to make workarounds, since getParam won't handle it as default value:

let optionalParam = this.props.navigation.getParam('optionalParam', '0')

if (optionalParam === 'undefined') {
  optionalParam = '0'
}

[ericvicenti]

Ah, this looks good