FIX: Fail if none of our tags could be updated

For example, if a category has a tag restriction and the API tries to
attempt to update it but cannot.

See:
https://meta.discourse.org/t/unallowed-tag-in-conversation-returns-200/122170

lib/discourse_tagging.rb

@@ -83,6 +83,7 @@ module DiscourseTagging
           return false
         end
 
+        return false if tags.size == 0
         topic.tags = tags
       else
         # validate minimum required tags for a category

spec/components/discourse_tagging_spec.rb

@@ -116,14 +116,29 @@ describe DiscourseTagging do
       other_category = Fabricate(:category, allowed_tags: [other_tag.name])
       topic = Fabricate(:topic, category: category)
 
-      DiscourseTagging.tag_topic_by_names(topic, Guardian.new(admin), [tag.name, other_tag.name, 'hello'])
+      result = DiscourseTagging.tag_topic_by_names(topic, Guardian.new(admin), [tag.name, other_tag.name, 'hello'])
+      expect(result).to eq(true)
       expect(topic.tags.pluck(:name)).to contain_exactly(tag.name)
 
       category.update!(allow_global_tags: true)
-      DiscourseTagging.tag_topic_by_names(topic, Guardian.new(admin), [tag.name, other_tag.name, 'hello'])
+      result = DiscourseTagging.tag_topic_by_names(topic, Guardian.new(admin), [tag.name, other_tag.name, 'hello'])
+      expect(result).to eq(true)
       expect(topic.tags.pluck(:name)).to contain_exactly(tag.name, 'hello')
     end
 
+    it 'raises an error if no tags could be updated' do
+      tag = Fabricate(:tag)
+      other_tag = Fabricate(:tag)
+      tag_group = Fabricate(:tag_group, permissions: { "staff" => 1 }, tag_names: [tag.name])
+      category = Fabricate(:category, allowed_tag_groups: [tag_group.name])
+      other_category = Fabricate(:category, allowed_tags: [other_tag.name])
+      topic = Fabricate(:topic, category: category)
+
+      result = DiscourseTagging.tag_topic_by_names(topic, Guardian.new(admin), [other_tag.name])
+      expect(result).to eq(false)
+      expect(topic.tags.pluck(:name)).to be_blank
+    end
+
     context 'respects category minimum_required_tags setting' do
       fab!(:category) { Fabricate(:category, minimum_required_tags: 2) }
       fab!(:topic) { Fabricate(:topic, category: category) }