Checklist

• I've run bundle exec rspec from the root directory to see all new and existing tests pass
• I've followed the fastlane code style and run bundle exec rubocop -a to ensure the code style is valid
• I've read the Contribution Guidelines
• I've updated the documentation if necessary.

Motivation and Context

Fixes #16480

Description

test_results used to early return when disable_xcpretty is true, but that would lead into a crash inside handle_test_results (the only method that calls test_results) because it doesn't check for nil. I'm changing the early return to be in handle_test_results instead so the whole thing is skipped.

Testing Steps

I believe any combination of disable_xcpretty: true without fail_build: false would lead into the crash before this fix. I used to have fail_build: true so I had never detected it myself.