Fix ShaAttribute concern when there is no table

When this is added to a new model, it would fail before the migrations were run
- including when trying to run migrations in production mode!

app/models/concerns/sha_attribute.rb

@@ -3,6 +3,8 @@ module ShaAttribute
 
   module ClassMethods
     def sha_attribute(name)
+      return unless table_exists?
+
       column = columns.find { |c| c.name == name.to_s }
 
       # In case the table doesn't exist we won't be able to find the column,

spec/models/concerns/sha_attribute_spec.rb

@@ -13,6 +13,11 @@ describe ShaAttribute do
   end
 
   describe '#sha_attribute' do
+    context' when the table exists' do
+      before do
+        allow(model).to receive(:table_exists?).and_return(true)
+      end
+
       it 'defines a SHA attribute for a binary column' do
         expect(model).to receive(:attribute)
           .with(:sha1, an_instance_of(Gitlab::Database::ShaAttribute))
@@ -24,4 +29,18 @@ describe ShaAttribute do
         expect { model.sha_attribute(:name) }.to raise_error(ArgumentError)
       end
     end
+
+    context' when the table does not exist' do
+      before do
+        allow(model).to receive(:table_exists?).and_return(false)
+      end
+
+      it 'does nothing' do
+        expect(model).not_to receive(:columns)
+        expect(model).not_to receive(:attribute)
+
+        model.sha_attribute(:name)
+      end
+    end
+  end
 end