Fix remaining failures in shoulda-matcher

Starting with Rails 5, belongs_to now adds a presence validation to the
association, and so as of shoulda-matchers 4.0.0 the belong_to matcher
follows suit and tests that this validation is there by setting the
association to nil and asserting that there are validation errors. This
exposed an error with the `validate_branches` method: we need to check
the source and target project exist.

app/models/merge_request.rb

@@ -581,6 +581,8 @@ class MergeRequest < ApplicationRecord
   end
   def validate_branches
+    return unless target_project && source_project
+
     if target_project == source_project && target_branch == source_branch
       errors.add :branch_conflict, "You can't use same project/branch for source and target"
       return

spec/models/deployment_spec.rb

@@ -5,8 +5,8 @@ require 'spec_helper'
 describe Deployment do
   subject { build(:deployment) }
-  it { is_expected.to belong_to(:project) }
-  it { is_expected.to belong_to(:environment) }
+  it { is_expected.to belong_to(:project).required }
+  it { is_expected.to belong_to(:environment).required }
   it { is_expected.to belong_to(:user) }
   it { is_expected.to belong_to(:deployable) }

spec/models/environment_spec.rb

@@ -6,7 +6,7 @@ describe Environment do
   let(:project) { create(:project, :stubbed_repository) }
   subject(:environment) { create(:environment, project: project) }
-  it { is_expected.to belong_to(:project) }
+  it { is_expected.to belong_to(:project).required }
   it { is_expected.to have_many(:deployments) }
   it { is_expected.to delegate_method(:stop_action).to(:last_deployment) }