Fix Error 500 when user commits Wiki page with no commit message

Previously if a user submitted a blank commit message, Gitaly would
error out with `3:WikiWritePage: empty CommitDetails.Message` since
Gitaly checks whether any content is present for the message. We now
use a default commit message if a user leaves it blank.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/59065

app/models/project_wiki.rb

@@ -183,7 +183,7 @@ class ProjectWiki
   end
 
   def commit_details(action, message = nil, title = nil)
-    commit_message = message || default_message(action, title)
+    commit_message = message.presence || default_message(action, title)
     git_user = Gitlab::Git::User.from_gitlab(@user)
 
     Gitlab::Git::Wiki::CommitDetails.new(@user.id,

changelogs/unreleased/sh-fix-issue-59065.yml

+---
+title: Fix Error 500 when user commits Wiki page with no commit message
+merge_request: 26247
+author:
+type: fixed

spec/models/project_wiki_spec.rb

@@ -71,6 +71,14 @@ describe ProjectWiki do
       expect(project_wiki.create_page("index", "test content")).to be_truthy
     end
 
+    it "creates a new wiki repo with a default commit message" do
+      expect(project_wiki.create_page("index", "test content", :markdown, "")).to be_truthy
+
+      page = project_wiki.find_page('index')
+
+      expect(page.last_version.message).to eq("#{user.username} created page: index")
+    end
+
     it "raises CouldNotCreateWikiError if it can't create the wiki repository" do
       # Create a fresh project which will not have a wiki
       project_wiki = described_class.new(create(:project), user)