Do not nil out the PR message for jobs updating a PR

[brrygrdn]

This fixes a bug introduced in #6792.

The newly introduced DependencyChange class made an assumption that any content created as result of a Job which had updating_a_pull_request? should not generate a pr_message object as the ApiClient historically did not accept a pr_message argument.

In reality, we have this scenario when updating_a_pull_request? is true to consider:

dependabot-core/updater/lib/dependabot/updater.rb

Lines 196 to 201 in a196e04

	job_dependencies = job.dependencies.map(&:downcase)
	if updated_deps.map(&:name).map(&:downcase) != job_dependencies
	# The dependencies being updated have changed. Close the existing
	# multi-dependency PR and try creating a new one.
	close_pull_request(reason: :dependencies_changed)
	create_pull_request(updated_deps, updated_files)

In a scenario where we close the existing PR and create a new one, the nil PR message blocks the API accepting the request.