I see some house fires were started at the outlet. They caught fire.
I don't know the circumstances on each case.

But can a outlet catch fire if nothing is plug into it??

I have had 1 outlet catch on fire -- believe it had a box fan plugged into it

Not a pretty sight but I believe the fan was actually the cause

In most cases it would need something plugged in to cause a fire

It may be possible for rodents to cause a fire -- not impossible

Maybe started a fire to keep warm -- just kidding

In most cases it would need something plugged in to cause a fireAbsolutely not!
If a connection is bad at a receptacle in the middle of the run something can be plugged in two rooms away and cause the receptacle to fail. I should make it clear that the connection will fail. Receptacles themselves rarely fail.
Backstab connections are infamous for this.

Think of your cuicuit as a single line diagram.The hot runs from plug to plug to light to plug ETC.if you have a bad connection after the pigtail(at each plug)then this would be more believable.

Loose Connections Generate heat and can cause fires, can be any loose connection(s), Where plug plugs into receptacle, where wires connect to receptacle, wire connections.
Loose or worn recepticles should be replaced to keep from damaging cord prongs.
As far as a Fire with nothing plugged in, if there is no load(and no heat), it can't start a fire unless a short or other problem, a lizzard or something could get between the Hot and the Box and May start something(smoking, Fire?)