Fix NoMethodError for empty changelog

Sentry error: https://sentry.gitlab.net/gitlab/gitlabcom/issues/3171592

Changelog: fixed

lib/gitlab/changelog/config.rb

@@ -37,7 +37,7 @@ module Gitlab
       attr_accessor :date_format, :categories, :template, :tag_regex, :always_credit_user_ids
 
       def self.from_git(project, user = nil)
-        if (yaml = project.repository.changelog_config)
+        if (yaml = project.repository.changelog_config.presence)
           from_hash(project, YAML.safe_load(yaml), user)
         else
           new(project)

spec/lib/gitlab/changelog/config_spec.rb

@@ -31,6 +31,20 @@ RSpec.describe Gitlab::Changelog::Config do
 
       described_class.from_git(project)
     end
+
+    context 'when changelog is empty' do
+      it 'returns the default configuration' do
+        allow(project.repository)
+          .to receive(:changelog_config)
+          .and_return("")
+
+        expect(described_class)
+          .to receive(:new)
+          .with(project)
+
+        described_class.from_git(project)
+      end
+    end
   end
 
   describe '.from_hash' do