[hist] remove default parameter in TH1::GetQuantiles, as is the case …

…with TF1::GetQuantiles

Fixes root-project#16784

The currently default parameter of p = nullptr is a very weird use case, which calculates F-1(F(bin_edges)), ie it just returns the bin edges. So force user to really decide if he wants to pass that nullptr as argument.

hist/hist/inc/TH1.h

@@ -304,7 +304,7 @@ class TH1 : public TNamed, public TAttLine, public TAttFill, public TAttMarker {
 
    TVirtualHistPainter *GetPainter(Option_t *option="");
 
-   virtual Int_t    GetQuantiles(Int_t n, Double_t *xp, const Double_t *p = nullptr);
+   virtual Int_t    GetQuantiles(Int_t n, Double_t *xp, const Double_t *p);
    virtual Double_t GetRandom(TRandom * rng = nullptr) const;
    virtual void     GetStats(Double_t *stats) const;
    virtual Double_t GetStdDev(Int_t axis=1) const;