[RF] Explicitly set empty normalization set in `RooGenProdProj`

The idea of the RooGenProdProj is that we divide two integral objects
each created with this makeIntgral() function to get the normalized
integral of a product. Therefore, we don't need to normalize the
numerater and denominator integrals themselves. Doing the normalization
would be expensive and it would cancel out anyway. However, if we don't
specify an explicit normalization integral in createIntegral(), the
last-used normalization set might be used to normalize the pdf,
resulting in redundant computations.

For this reason, the normalization set of the integrated pdfs is fixed
to an empty set in this case. Note that in RooFit, a nullptr
normalization set and an empty normalization set is not equivalent. The
former implies taking the last-used normalization set, and the latter
means explicitly no normalization.

This fixes the performance regression reported in #11814, and a new unit
test is implemented to make sure no new numeric integrals pop up in the
reproducer code to that issue.

Unfortunately, this change means that there will be again warnings about
missing `RooAddPdf` normalization sets, but this is not a regression
because these warnings only got fixed in the 6.28 development cycle in
290b4787.