Avoid depositing negative charge on sTGC strips

By convention, charge placing on sTGC strips is positive. However, when the incident angle approaches 90 degrees, the standard deviation that depends on tan(theta) becomes as big as (or even bigger than) the mean, resulting in negative charge sometimes. It is a minor bug. This merge request adds simply a workaround to set charge to zero when it is negative charge. A permanent fix is being tested, but tuning parameters is time consuming. Also, MR !51362 (merged) reduces a large fraction of hits that likely induce negative charge on strips.