Fix enviroments seeding on certain branch names

Bit of a cryptic commit title, but our seeding will try to guard against
branchnames being anything but alphanumerical. These charachters we gsub
away but can't have an empty string as replacement.

This change is less efficient, but given that the number of seeded MRs
per project is very limited, this should be ok.

db/fixtures/development/19_environments.rb

@@ -28,7 +28,11 @@ class Gitlab::Seeder::Environments
   end
 
   def create_merge_request_review_deployments!
-    @project.merge_requests.sample(4).map do |merge_request|
+    @project
+      .merge_requests
+      .select { |mr| mr.source_branch.match(/\p{Alnum}+/) }
+      .sample(4)
+      .each do |merge_request|
       next unless merge_request.diff_head_sha
 
       create_deployment!(