[Trie] Fix trie contains function for empty string

[lexorus]

Checklist

• I've looked at the contribution guidelines.
• This pull request is complete and ready for review.

Description

Currently Trie().contains(word: "", matchPrefix: true)) will return false, which is wrong since the root node of a Trie is considered an empty string. This PR fixes this bug. More information can be found here or by checking implementations of Trie data structure in other languages.
There is actually already a test written for this, which is failing at the moment. So this PR also makes tests for Trie data structure to pass.

What has been done

1. Fix contains function to return true for empty string
2. Update Trie project to Swift 5. No code changes were required.
3. Removed useless UI tests.

How to test

Open Trie.xcodeproj and make sure project builds and tests are passing.