detselect should treat NULL values as always matching

[eugene]

in detselect, it seems that a NULL value in one of the restriction fields (eg, exp_time_max) is treated as a 0.0 value. this may work for the minima, but it does not work for the maxima. If, for example, we search by exptime, then we get the constraints:
exp_time_min < exptime + epsilon AND
exp_time_max > exptime + epsilon

if the detRun has exp_time_max set to NULL, the second condition is never true. the logic should instead be something like:

((exp_time_min IS NULL) OR (exp_time_min < exptime + epsilon)) AND
((exp_time_max IS NULL) OR (exp_time_max > exptime + epsilon))

[jhoblitt]

i beleive this has been dealt with