GithubImporter: Avoid failing when PullRequest has empty body

Related to:
- https://gitlab.com/gitlab-org/gitlab/-/issues/334203
- https://sentry.gitlab.net/gitlab/staginggitlabcom/issues/2693029

Changelog: fixed
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/64558

lib/gitlab/github_import/markdown_text.rb

@@ -13,7 +13,7 @@ module Gitlab
       # author - An instance of `Gitlab::GithubImport::Representation::User`
       # exists - Boolean that indicates the user exists in the GitLab database.
       def initialize(text, author, exists = false)
-        @text = text
+        @text = text.to_s
         @author = author
         @exists = exists
       end

spec/lib/gitlab/github_import/markdown_text_spec.rb

@@ -27,6 +27,13 @@ RSpec.describe Gitlab::GithubImport::MarkdownText do
       expect(text.to_s).to eq('Hello')
     end
 
+    it 'returns empty text when it receives nil' do
+      author = double(:author, login: nil)
+      text = described_class.new(nil, author, true)
+
+      expect(text.to_s).to eq('')
+    end
+
     it 'returns the text with an extra header when the author was not found' do
       author = double(:author, login: 'Alice')
       text = described_class.new('Hello', author)