A tricky one indeed. In theory it should not do any damage but leaving any unused electrical appliance connected to the live mains is not recommended.
The type and amperages of the battery and charger also has a bearing on any potential damage. Leaving an inactive (unplugged) charger connected to your laptop if it has a battery in it is certainly not recommended.
Sorry I can't be more definite. lets hope not all the experts have gone on holiday and you get a qualified answer.
1 Most laptop have a controlled charger so the theory is that it switches off when the battery is full .....
however
Lilon batteries store best when only 25% charged .... so there is an argument to taking the battery out when not on the move. (however the battery does form part of the case .... and so taking it out may result in the case flexing in a bad way (some machines come with blank pieces.
the other is (almost) mechanical
2 When charging ... or when on the charger ... the power transistor can get extremely hot ....(feel and see if this happens)
I've seen laptops placed on polished surfaces create quite bad damage to some quite expensive furniture!!!
trhis heat can also "fry" the battery (which contains water) reducing the life of the battery.
finally .... from a green point of view .... leaving power on ..... istn't