Provide sensible default values for the inputs

[cz4rs]

We could provide sensible defaults for most of the inputs:

comment-on-pr/action.yml

Lines 5 to 28 in 0238c66

	repo_owner:
	description: 'Repository owner name'
	required: true
	default: ''
	repo_name:
	description: 'Repository name'
	required: true
	default: ''
	pr_number:
	description: 'PR number'
	required: true
	default: ''
	comment_title:
	description: 'Title of the comment (used also for searching of possibly existing comment)'
	required: true
	default: ''
	comment_content:
	description: 'Content of the comment'
	required: true
	default: ''
	github_token:
	description: 'GITHUB_TOKEN'
	required: true
	default: ''