fix: handle nil Vars in ToCacheMap (#2842)

taskfile/ast/vars.go

@@ -98,6 +98,9 @@ func (vars *Vars) Values() iter.Seq[Var] {
 // ToCacheMap converts Vars to an unordered map containing only the static
 // variables
 func (vars *Vars) ToCacheMap() (m map[string]any) {
+	if vars == nil || vars.om == nil {
+		return nil
+	}
 	defer vars.mutex.RUnlock()
 	vars.mutex.RLock()
 	m = make(map[string]any, vars.Len())