Return empty strings for Jira links when URL is not set

With the changes in cb4de493 we started returning `nil` from methods
like `Integrations::Jira#issues_url` if the URL is not set, which broke
certain pages like the project dashboard.

Changelog: fixed

app/models/integrations/jira.rb

@@ -155,7 +155,7 @@ module Integrations
     end
 
     def web_url(path = nil, **params)
-      return unless url.present?
+      return '' unless url.present?
 
       if Gitlab.com?
         params.merge!(ATLASSIAN_REFERRER_GITLAB_COM) unless Gitlab.staging?

spec/models/integrations/jira_spec.rb

@@ -1024,6 +1024,12 @@ RSpec.describe Integrations::Jira do
         expect(integration.web_url('subpath', bar: 'baz baz')).to eq('http://jira.test.com/path/subpath?bar=baz%20baz&foo=bar%20bar&nosso')
       end
 
+      it 'returns an empty string if URL is not set' do
+        integration.url = nil
+
+        expect(integration.web_url).to eq('')
+      end
+
       it 'includes Atlassian referrer for gitlab.com' do
         allow(Gitlab).to receive(:com?).and_return(true)
 
@@ -1041,16 +1047,40 @@ RSpec.describe Integrations::Jira do
       end
     end
 
+    describe '#project_url' do
+      it 'returns the correct URL' do
+        expect(integration.project_url).to eq('http://jira.test.com/path')
+      end
+
+      it 'returns an empty string if URL is not set' do
+        integration.url = nil
+
+        expect(integration.project_url).to eq('')
+      end
+    end
+
     describe '#issues_url' do
       it 'returns the correct URL' do
         expect(integration.issues_url).to eq('http://jira.test.com/path/browse/:id')
       end
+
+      it 'returns an empty string if URL is not set' do
+        integration.url = nil
+
+        expect(integration.issues_url).to eq('')
+      end
     end
 
     describe '#new_issue_url' do
       it 'returns the correct URL' do
         expect(integration.new_issue_url).to eq('http://jira.test.com/path/secure/CreateIssue!default.jspa')
       end
+
+      it 'returns an empty string if URL is not set' do
+        integration.url = nil
+
+        expect(integration.new_issue_url).to eq('')
+      end
     end
   end