Search for in ("") raises strptime() error

[danmoz]

Reproduction steps:

1. Go to your UserAdmin
2. Search for created_at in ("")

Exception is raised:

TypeError at /admin/user/user/
strptime() argument 1 must be str, not list

Expected behaviour:

Exception is handled as if the user had searched for any other invalid date, e.g.

created_at in ("foo")

which handles the error with a message:

Field "created_at" can be compared to timestamps in "YYYY-MM-DD HH:MM" format, but not to 'foo'