Validate result publication enabled processes have a date

Just like we do with the rest of the phases.

The reason why we're making this change right now is that we were
getting an accessibility error with processes with no result publication
date:

```
link-name: Links must have discernible text (serious)
https://dequeuniversity.com/rules/axe/4.10/link-name?application=axeAPI
The following 1 node violate this rule:

  Selector: p:nth-child(6) > a
  HTML: <a href="/legislation/processes/39/result_publication">
          <strong></strong>
        </a>
  Fix all of the following:
  - Element is in tab order and does not have accessible text
  Fix any of the following:
  - Element does not have text that is visible to screen readers
  - aria-label attribute does not exist or is empty
  - aria-labelledby attribute does not exist, references elements that
    do not exist or references elements that are empty
  - Element has no title attribute
```

app/models/legislation/process.rb

@@ -46,6 +46,7 @@ class Legislation::Process < ApplicationRecord
   validates_translation :title, presence: true
   validates :start_date, presence: true
   validates :end_date, presence: true
+  validates :result_publication_date, presence: true, if: :result_publication_enabled?
 
   %i[draft debate proposals_phase allegations].each do |phase_name|
     enabled_attribute = :"#{phase_name.to_s.gsub("_phase", "")}_phase_enabled?"

spec/models/legislation/process_spec.rb

@@ -154,6 +154,21 @@ describe Legislation::Process do
       expect(proposals_disabled).to be_valid
       expect(allegations_disabled).to be_valid
     end
+
+    it "is invalid if result publication is enabled but result_publication_date is not present" do
+      process = build(:legislation_process, result_publication_enabled: true, result_publication_date: "")
+
+      expect(process).not_to be_valid
+      expect(process.errors.messages[:result_publication_date]).to include("can't be blank")
+    end
+
+    it "is valid if result publication is enabled and result_publication_date is present" do
+      process = build(:legislation_process,
+                      result_publication_enabled: true,
+                      result_publication_date: Date.tomorrow)
+
+      expect(process).to be_valid
+    end
   end
 
   describe "date ranges validations" do

spec/system/legislation/summary_spec.rb

@@ -22,6 +22,7 @@ describe "Legislation" do
   scenario "empty process" do
     process = create(:legislation_process, :empty,
                      result_publication_enabled: true,
+                     result_publication_date: 1.day.ago,
                      end_date: Date.current - 1.day)
 
     visit summary_legislation_process_path(process)