Added nil check to avoid exception when production track is not present

[stivlo]

With the original code if production track is not present, we get the following exception:

/fastlane-2.220.0/supply/lib/supply/client.rb:303:in block in latest_version': undefined method name' for nil (NoMethodError)

  latest_version = tracks.select { |t| t.track == Supply::Tracks::DEFAULT }.map(&:releases).flatten.reject { |r| r.name.nil? }.max_by(&:name)
                                                                                                                  ^^^^^

with the modified code we get the correct error message:

[!] Unable to find latest version information from "production" track. Please specify track information by using the '--track' option.

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

Description

Testing Steps