[RF] Don't override isReducerNode() in RooPolynomial

This was an unneeded optimization, because if the Polynomial doesn't
depend on any coefficients and is constant, its results will be cached
anyway so it doesn't cost much to fill the full ouput array.

This fixes problems with the RooFit code generation facility that uses
`isReducerNode()` for scope management.

Author: guitargeek

roofit/roofit/inc/RooPolynomial.h

@@ -44,10 +44,6 @@ class RooPolynomial : public RooAbsPdf {
    /// Return the order for the first coefficient in the list.
    int lowestOrder() const { return _lowestOrder; }
 
-   // If this polynomial has no terms it's a uniform distribution, and a uniform
-   // pdf is a reducer node because it doesn't depend on the observables.
-   bool isReducerNode() const override { return _coefList.empty(); }
-
 protected:
    RooRealProxy _x;
    RooListProxy _coefList;

roofit/roofitcore/src/RooPolyVar.cxx

@@ -116,7 +116,9 @@ void RooPolyVar::doEvalImpl(RooAbsArg const *caller, RooFit::EvalContext &ctx, R
 {
    std::span<double> output = ctx.output();
    if (coefs.empty()) {
-      output[0] = lowestOrder ? 1.0 : 0.0;
+      for (std::size_t i = 0; i < output.size(); ++i) {
+         output[i] = lowestOrder ? 1.0 : 0.0;
+      }
       return;
    }