org-insert-mode-line-in-empty-file

org-insert-mode-line-in-empty-file is a variable defined in `org.el'.
Its value is

nil

Documentation:
Non-nil means insert the first line setting Org-mode in empty files.
When the function `org-mode' is called interactively in an empty file, this
normally means that the file name does not automatically trigger Org-mode.
To ensure that the file will always be in Org-mode in the future, a
line enforcing Org-mode will be inserted into the buffer, if this option
has been set.

You can customize this variable.