Fix checking target branch and label name.

[M-Yamashita01]

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We found that it is not raised error from merge-branch action when target_branch and label_name are empty strings.

When we set empty strings to the target_branch in yaml file and executed GitHub Actions in the personal repository, the Actions failed. https://github.com/M-Yamashita01/RubySample/runs/4042641757?check_suite_focus=true

And when we set empty strings to the label_name in yaml file and executed GitHub Actions, the error should have been raised, but the Actions passed it by mistake. https://github.com/M-Yamashita01/RubySample/runs/4051426151?check_suite_focus=true

Therefore, we fixed them and added rspec gem and tests.