Count number of PRs open in CD-enabled apps before deciding string

It's quite possible that there are CD-enabled apps managed by the
given team, but that there are no PRs open for them. Currently, it
will say:

```
[TEAM_NAME] have 0 Dependabot PRs open on the following
Continuously Deployed apps:

And 73 Dependabot PRs open on other apps:
```

Screenshot:

![](https://user-images.githubusercontent.com/5111927/107197652-10346680-69ec-11eb-926d-64b8f02f74a5.png)

This commit now counts the number of PRs in those apps before
deciding whether to show the CD part of the string.

lib/presenters/slack/full_message.rb

@@ -3,8 +3,8 @@ module Slack
     class FullMessage
       def execute(applications_by_team:, continuously_deployed_apps: [])
         applications = applications_by_team.fetch(:applications)
-        if continuously_deployed_apps.any?
-          cd_apps = applications.filter { |app| continuously_deployed_apps.include?(app.fetch(:application_name)) }
+        cd_apps = applications.filter { |app| continuously_deployed_apps.include?(app.fetch(:application_name)) }
+        if pull_requests_count(cd_apps).positive?
           "#{url_for_team(applications_by_team)} have #{pull_requests_count(cd_apps)} Dependabot PRs open on the following Continuously Deployed apps:
 
 #{body(cd_apps).join(' ')}