[RF] Handle nullptr or empty norm set ambiguity in RooAbsReal::getVal
[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 #8307.

olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root