iRule to drop HTTP GET requests that don't have zero content length header

Are you looking for requests that have any content length (like a POST):

when HTTP_REQUEST {
    if { [HTTP::header exists Content-Length] } {
        drop
    }
}

Or something that specifically has a content length of zero:

when HTTP_REQUEST {
    if { ( [HTTP::header exists Content-Length] ) and ( [HTTP::header Content-Length] == 0 ) } {
        drop
    }
}