Solar panels at night without diodes - short circuit or source of power dissipation?
I assume you mean "open circuit or source of power dissipation" - if it was short circuit it would cause substantial power dissipation.

Summary: It is usual to fit blocking diodes and good practice but on small to medium panels at voltages under say 100V it is probably not strictly necessary. 

Detail:

It is usual to fit blocking diodes to prevent reverse current when Vbattery is > Vpanel.

I have an experimental setup here with a 30Vmp 250 Wmp panel so Imp is about I=W/V ~= 8.3A. Those are the values you would get in full sun (insolation about 1 sun = 1000 W/m^2). It's 10:45pm here at present and the panel is in near complete darkness. I just applied 25VDC to the panel and it drew 40 mA or about 0.04/8.3 ~= 0.5% of full current. That is not a trivial amount but may be acceptable. In a typical day with say 4 sunshine hours the % current lost at that rate is in the order of say 12 hours x 40 mA / (4 hours x 8.3A) =~ 1.5% of the day's insolation wasted. The current lost will be higher at the dark to day transitions but still probably modest - see below. 

In practice the ability of the panel to pass current relates to the current that it would provide if optimally loaded. Consider maximum power = Wmp to occur at 1 sun of insolation = 1000 W/m^2. For much of the light level range from about 5% x 1 sun or less up to 1 sun the panel is close to a current source with current close to being proportional to light level and Vout being about 70% of max output at low light levels and close to Vmp at most light levels above 10%. Vpanel only falls substantially at say 1% x 1 sun or less. 

The implications of the above is that when Vpanel is well below battery voltage the current it will support is minimal and in the dark it is about zero. While a blocking diode will prevent this loss it also has potential losses of its own. A schottky diode will drop say 0.4V to 0.6v in smaller systems depending on load and more than that in large systems. On a say 12V battery system Vmp is typically 18V (which is probably higher than needed for most situations). A fully charged floated lead acid battery requires about 13.7 V and you "never" need over 15V so a diode drop is not too problematic except in VERY low light conditions. 

At very high voltages (hundreds of volts) you can get cell breakdown in some cases and diodes seem like a good idea. Also eg 40 mA x 1000V = 40W - not to be sneezed at.