IntelliJ IDEA doesn’t pre-select author in the combobox which is a known cosmetic issue, however it should use the author defined in the git configuration by default.
Please double check that user.name and user.email is specified properly in the global git configuration.
Some users mentioned a problem when using cygwin git, when IDEA probably cannot find the global git config. In this case it may help if you specify the user for the local project configuration:
git config --local user.name "John Doe"
git config --local user.email john@doe.com