Bump cel-go to v0.17.6

vendor/github.com/google/cel-go/interpreter/evalstate.go

@@ -66,7 +66,11 @@ func (s *evalState) Value(exprID int64) (ref.Val, bool) {
 
 // SetValue is an implementation of the EvalState interface method.
 func (s *evalState) SetValue(exprID int64, val ref.Val) {
-	s.values[exprID] = val
+	if val == nil {
+		delete(s.values, exprID)
+	} else {
+		s.values[exprID] = val
+	}
 }
 
 // Reset implements the EvalState interface method.