In CE, this does nothing - the `MergeRequests::BuildService` will, at the time
of writing, never return a description for this case.
In EE, a project can have a default MR template, which will be returned by the
service. Previously we were only using the description passed in the params,
ignoring any already set on the object. Now we fall back to the one set on the
object if there was none in the params, allowing quick actions to be executed
from default MR templates when creating an MR from an issue.
