Fix Crash in Scan Runner.rb

[cpisciotta]

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 see several green ci/circleci builds in the "All checks have passed" section of my PR (connect CircleCI to GitHub if not)
• I've read the Contribution Guidelines
• I've updated the documentation if necessary.
• I've added or updated relevant unit tests.

Motivation and Context

Resolves a crash when Scan.devices is nil.

Description

Similar to the lines above it, check if Scan.devices is non-nil before iterating over its values.

Testing Steps