This is certainly a somewhat odd choice, but from a functional perspective not important.
Since this is a static variable, it is simply set to 0 initially, and reset at initialization (function InitCool), either to 0.1 the min. cooling temperature in the parameter file or to 1.
So the zero is never actually used. 1, i.e. 10 K is a fairly common lower limit for galaxy scale simulations (many have far higher temperature limits, in fact).
I agree with you it might be a bit strange, but not really of any consequence.