Simplify basic presence validations (#29664)

spec/models/media_attachment_spec.rb

@@ -257,12 +257,7 @@ RSpec.describe MediaAttachment, :attachment_processing do
     end
   end
 
-  it 'is invalid without file' do
-    media = described_class.new
-
-    expect(media.valid?).to be false
-    expect(media).to model_have_error_on_field(:file)
-  end
+  it { is_expected.to validate_presence_of(:file) }
 
   describe 'size limit validation' do
     it 'rejects video files that are too large' do