org-sort-agenda-noeffort-is-high

org-sort-agenda-noeffort-is-high is a variable defined in `org-agenda.el'.
Its value is

t

Documentation:
Non-nil means items without effort estimate are sorted as high effort.
This also applies when filtering an agenda view with respect to the
< or > effort operator. Then, tasks with no effort defined will be treated
as tasks with high effort.
When nil, such items are sorted as 0 minutes effort.

You can customize this variable.