FIX: Prevent saving empty string as a locale (#33481)

Post edits need to inform the backend if the locale is changed and is
sometimes set as `""`. This PR prevents the locale from being set to
`""` in the database, and includes a migration to purge the empty
strings.

app/assets/javascripts/discourse/app/components/post-language-selector.gjs

@@ -59,7 +59,7 @@ export default class PostLanguageSelector extends Component {
           <dropdown.item>
             <DButton
               @label="post.localizations.post_language_selector.none"
-              @action={{fn this.selectPostLanguage ""}}
+              @action={{fn this.selectPostLanguage null}}
             />
           </dropdown.item>
         </DropdownMenu>

app/models/post.rb

@@ -888,6 +888,8 @@ class Post < ActiveRecord::Base
       self.baked_at = Time.zone.now
       self.baked_version = BAKED_VERSION
     end
+
+    self.locale = nil if locale.blank?
   end
 
   def advance_draft_sequence

app/models/topic.rb

@@ -415,6 +415,8 @@ class Topic < ActiveRecord::Base
       inherit_auto_close_from_category
       inherit_slow_mode_from_category
     end
+
+    self.locale = nil if locale.blank?
   end
 
   after_save do

db/migrate/20250704072726_nullify_blank_locales.rb

@@ -0,0 +1,28 @@
+# frozen_string_literal: true
+class NullifyBlankLocales < ActiveRecord::Migration[7.2]
+  def up
+    loop do
+      result = execute(<<~SQL)
+          UPDATE topics
+          SET locale = NULL
+          WHERE id IN (SELECT id FROM topics WHERE locale = '' LIMIT 5000)
+        SQL
+
+      break if result.cmd_tuples == 0
+    end
+
+    loop do
+      result = execute(<<~SQL)
+          UPDATE posts
+          SET locale = NULL
+          WHERE id IN (SELECT id FROM posts WHERE locale = '' LIMIT 5000)
+        SQL
+
+      break if result.cmd_tuples == 0
+    end
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end

spec/models/post_spec.rb

@@ -2409,4 +2409,15 @@ RSpec.describe Post do
       expect(post.in_user_locale?).to eq(false)
     end
   end
+
+  describe "#before_save" do
+    it "replaces empty locales with nil" do
+      post = Fabricate(:post, locale: "en")
+
+      post.locale = ""
+      post.save!
+
+      expect(post.reload.locale).to eq(nil)
+    end
+  end
 end

spec/models/topic_spec.rb

@@ -3332,6 +3332,17 @@ describe Topic do
     end
   end
 
+  describe "#before_save" do
+    it "replaces empty locales with nil" do
+      topic = Fabricate(:topic, locale: "en")
+
+      topic.locale = ""
+      topic.save!
+
+      expect(topic.reload.locale).to eq(nil)
+    end
+  end
+
   describe "#after_update" do
     fab!(:topic) { Fabricate(:topic, user: user) }
     fab!(:category) { Fabricate(:category_with_definition, read_restricted: true) }