navigation.goBack(null) and fix SearchBar cancelButtonText

[alangumer]

• Add null parameter, If null parameter is not present it won't go back when the Search Screen is a part of a top-level Navigator.
• Fix SearchBar cancelButtonText.
• Fix HeaderBackButton import

[brentvatne]

i think at this point this library needs to be updated for react-navigation v5 now :)

[eriveltonelias]

@brentvatne yeah :), I'm planning to do that next week.

[armenio]

i think at this point this library needs to be updated for react-navigation v5 now :)

https://github.com/react-navigation/search-layout/pull/20