Allow compound blocks without child blocks

This is useful in some cases, for example with hero blocks where we
might only want the image and no content overlaid.

Probably not so useful for featured blocks, but as we're not using those
right now, I think it's fine for them not to have this validation.

app/models/landing_page/compound_block.rb

@@ -18,7 +18,6 @@ class LandingPage::CompoundBlock < LandingPage::BaseBlock
 
   attr_reader :content_blocks, :content_block_key
 
-  validates :content_blocks, presence: true
   validate do
     content_blocks.each { |b| errors.merge!(b.errors) if b.invalid? }
   end
@@ -30,6 +29,10 @@ def initialize(source, images, content_block_key, content_blocks)
   end
 
   def present_for_publishing_api
-    super.merge({ content_block_key => { "blocks" => content_blocks.map(&:present_for_publishing_api) } })
+    if content_blocks.blank?
+      super
+    else
+      super.merge({ content_block_key => { "blocks" => content_blocks.map(&:present_for_publishing_api) } })
+    end
   end
 end

test/unit/app/models/landing_page/compound_block_test.rb

@@ -37,15 +37,24 @@ class CompoundBlockTest < ActiveSupport::TestCase
     assert_equal(expected_result, subject.present_for_publishing_api)
   end
 
-  test "invalid when missing content blocks" do
+  test "valid when missing content blocks" do
     subject = LandingPage::CompoundBlock.new(
       @valid_block_config,
       EMPTY_IMAGES,
       "compound_block_content",
       nil,
     )
-    assert subject.invalid?
-    assert_equal ["Content blocks can't be blank"], subject.errors.to_a
+    assert subject.valid?
+  end
+
+  test "presents without missing content blocks" do
+    subject = LandingPage::CompoundBlock.new(
+      @valid_block_config.except("compound_block_content"),
+      EMPTY_IMAGES,
+      "compound_block_content",
+      nil,
+    )
+    assert_equal({ "type" => "some-compound-block" }, subject.present_for_publishing_api)
   end
 
   test "invalid when content blocks are invalid" do

test/unit/app/models/landing_page/featured_block_test.rb

@@ -68,10 +68,9 @@ class FeaturedBlockTest < ActiveSupport::TestCase
     ], subject.errors.to_a
   end
 
-  test "invalid when missing featured content blocks" do
+  test "valid when missing featured content blocks" do
     subject = LandingPage::FeaturedBlock.new(@valid_featured_block_config, @valid_featured_images, nil)
-    assert subject.invalid?
-    assert_equal ["Content blocks can't be blank"], subject.errors.to_a
+    assert subject.valid?
   end
 
   test "invalid when featured content blocks are invalid" do

test/unit/app/models/landing_page/hero_block_test.rb

@@ -69,10 +69,9 @@ class HeroBlockTest < ActiveSupport::TestCase
     ], subject.errors.to_a
   end
 
-  test "invalid when missing hero content blocks" do
+  test "valid when missing hero content blocks" do
     subject = LandingPage::HeroBlock.new(@valid_hero_block_config, @valid_hero_block_images, nil)
-    assert subject.invalid?
-    assert_equal ["Content blocks can't be blank"], subject.errors.to_a
+    subject.valid?
   end
 
   test "invalid when hero content blocks are invalid" do