Charge a single AA battery in a double charger?
If the charger is a genuine slow trickle charger then one battery in place of two batteries in series may well be OK. 

Otherwise, shorting out one battery position will have variable results.

This may scramble the few neurons available to simple battery charger controllers and mean that the battery never charges or over charges and may not stop charging. Clamping a two battery power supply to a one battery voltage will probably cause more current than intended unless they saw you coming and took due precautions. 

Two batteries in series is not a good match to some of the best charge termination methods. Often used is negative delta V where terminal voltage at constant current drops slightly at fully-charged. With two batteries in series this "signal" is more likely to be missed. 

One possibility is to use an oldish deadish or low cost battery in the second position. This should not be so dead as to read "high" under charge but need not have the capacity of the good battery.

A diode string that drops about 1.4V when used in place of the second battery may work OK. Use of 2 x 1N400x diodes MAY be OK. Maybe a 1N400x + a Schottky + ...? A zener or clamp regulator that drops 1.4V may work. 



Charging a NiCd battery in a NimH charger may work and may not. If you are able to measure battery voltage under charge and stop charging at about >= 1.4V  it will work OK (probably).

Lithium primary batteries must not be recharged.

Lithium secondary batteries MUST NOT be charged in any charger not designed for them, that their days (and yours)(and the charger's) may be long on the face of the land.