[RF] Handle nullptr or empty norm set ambiguity in RooAbsReal::getVal

The downsteam code, -- like RooAddPdf::getValV for example -- assume
that a nullptr is passed when no normalization is requested. The case of
an empty norm set is not handled correctly in RooAddPdf::getValV,
leading to wrong results.

However, some calling code passes an empty norm set to
RooAbsReal::getVal instead of a `nullptr` in an attempt to disable
normalization.

This commit suggests to solve this ambiguity at the highest possible
level: right at the beginning of RooAbsReal::getVal. If the
normalization set is empty, the pointer pointing to it will be set to
`nullptr`.

This fixes issue root-project#8307.

roofit/roofitcore/inc/RooAbsReal.h

@@ -89,6 +89,14 @@ class RooAbsReal : public RooAbsArg {
   /// These are integrated over their current ranges to compute the normalisation constant,
   /// and the unnormalised result is divided by this value.
   inline Double_t getVal(const RooArgSet* normalisationSet = nullptr) const {
+    // Sometimes, the calling code uses an empty RooArgSet to request evaluation
+    // without normalization set instead of following the `nullptr` convention.
+    // To remove this ambiguity which might not always be correctly handled in
+    // downstream code, we set `normalisationSet` to nullptr if it is pointing
+    // to an empty set.
+    if(normalisationSet && normalisationSet->empty()) {
+      normalisationSet = nullptr;
+    }
 #ifdef ROOFIT_CHECK_CACHED_VALUES
     return _DEBUG_getVal(normalisationSet);
 #else
@@ -103,7 +111,13 @@ class RooAbsReal : public RooAbsArg {
   }
 
   /// Like getVal(const RooArgSet*), but always requires an argument for normalisation.
-  inline  Double_t getVal(const RooArgSet& normalisationSet) const { return _fast ? _value : getValV(&normalisationSet) ; }
+  inline  Double_t getVal(const RooArgSet& normalisationSet) const {
+    // Sometimes, the calling code uses an empty RooArgSet to request evaluation
+    // without normalization set instead of following the `nullptr` convention.
+    // To remove this ambiguity which might not always be correctly handled in
+    // downstream code, we set `normalisationSet` to nullptr if it is an empty set.
+    return _fast ? _value : getValV(normalisationSet.empty() ? nullptr : &normalisationSet) ;
+  }
 
   virtual Double_t getValV(const RooArgSet* normalisationSet = nullptr) const ;