22.0-FPE_overflow_fix-TrigBphysMonitoring

Fix for [ATR-20518]; leaving the vertex covariance matrix undefined (in the case that evtInfo is null) at line 142 of the BphysTrigDiMuDecoratorTool resulted, strangely, in an FPE OVERFLOW (some peculiarity of eigen?) in the succeeding lines in opt build (but not dbg). Ensuring it is defined solves this.