Properly fix seeding empty database

Ref: bef7e6dc

db/seeds.rb

@@ -8,7 +8,7 @@
 #     MovieGenre.find_or_create_by!(name: genre_name)
 #   end
 
-# Conference.find_or_create_by!(slug: "rp2023") do |c|
+# Conference.find_or_create_by(slug: "rp2023").tap do |c|
 #   c.name = "re:publica 2023"
 #   c.time_zone = "Berlin"
 #   c.starts_at = DateTime.parse("Mon, 05 Jun 2023 08:30:00.000000000 UTC +00:00")
@@ -22,7 +22,7 @@
 #   c.save!
 # end
 
-# Conference.find_or_create_by!(slug: "rp2024") do |c|
+# Conference.find_or_create_by(slug: "rp2024").tap do |c|
 #   c.name = "re:publica 2024"
 #   c.time_zone = "Berlin"
 #   c.starts_at = DateTime.parse("27 May 2024 10:30 CEST")
@@ -36,7 +36,7 @@
 #   c.save!
 # end
 
-# Conference.find_or_create_by!(slug: "37c3") do |c|
+# Conference.find_or_create_by(slug: "37c3").tap do |c|
 #   c.name = "37th Chaos Communication Congress"
 #   c.time_zone = "Berlin"
 #   c.starts_at = DateTime.parse("27 December 2023 10:30 CET")
@@ -49,7 +49,7 @@
 #   c.save!
 # end
 
-Conference.find_or_create_by!(slug: "38c3").tap do |c|
+Conference.find_or_create_by(slug: "38c3").tap do |c|
   c.name = "38th Chaos Communication Congress (de-en)"
   c.time_zone = "Berlin"
   c.starts_at = DateTime.parse("27 December 2024 10:30 CET")
@@ -66,7 +66,7 @@ Conference.find_or_create_by!(slug: "38c3").tap do |c|
   c.save!
 end
 
-Conference.find_or_create_by!(slug: "38c3-more").tap do |c|
+Conference.find_or_create_by(slug: "38c3-more").tap do |c|
   c.name = "38th Chaos Communication Congress (more languages)"
   c.time_zone = "Berlin"
   c.starts_at = DateTime.parse("27 December 2024 10:30 CET")