Incorrect message when `only: merge_requests` missing from YAML using "Pipelines for Merge Requests" feature

Summary

Trying to demo the "Pipelines for Merge Requests" feature, I turned on the "Merge pipelines will try to validate the post-merge result prior to merging" checkbox, but I didn't have any jobs with only: merge_requests set. The end result was that my MRs couldn't be merged. The MRs show the message "You can only merge once the above items are resolved" but there are no "items" above that need to be merged. It seems like the messaging here should be more accurate.

Steps to reproduce

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Example Project

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What is the current bug behavior?

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What is the expected correct behavior?

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Relevant logs and/or screenshots

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Output of checks

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Results of GitLab environment info

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sudo gitlab-rake gitlab:env:info)
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sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production)

Results of GitLab application Check

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sudo gitlab-rake gitlab:check SANITIZE=true)
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sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes

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/cc @brendan