Removes null checks when possible

[Ascenio]

Closes #4.

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• The GitHub Actions pass building and linting. Linter returns no warnings or errors.
• The QA checklist below has been completed

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

There are lots of null checks, mainly in SessionController. We should try to minimize that.

Issue Number: #4

What is the new behavior?

Where there were default values there's no need for null check anymore.

Does this introduce a breaking change?

• Yes
• No

QA Checklist

UIKit Update Checklist (Minor or Patch Release)

• Using the latest version of Agora's Video SDK
• Example apps are all functional
• Core features are still working (both ways across platforms)
  • Camera + Mic muting works for local and remote users
  • Users are added and removed correctly when they join and leave the channel
  • Older versions of the library gracefully handle changes (Create issue if not)
  • Builtin buttons all work as expected
• Any newly deprecated methods are flagged as such inline and in documentation

UIKit Update Checklist (Major Release)

• The above checklist is completed (except backwards compatibility)
• Thoroughly tested for crashes, across multiple platforms at the same time