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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def find_project_config_file(project_root: str) -> str: """Return absolute path to project-specific config file, if it exists. :param project_root: Absolute path to project root directory. A project config file is a file named `YCONFIG_FILE` found at the top level of the project root dir. Return `None` if project root dir is not specified, or if no such file is found. """

    if project_root:
        project_config_file = os.path.join(project_root, YCONFIG_FILE)
        if os.path.isfile(project_config_file):
            return project_config_file