Bug in TKDE::Fill

[lmoneta]

Using TKDE::Fill on an empty TKDE does not work.
See https://root-forum.cern.ch/t/using-tkde-fill/44315

Example code to reproduce it:

auto kde = new TKDE(0, nullptr, 0, 5, "", 1); // or variations
for (unsigned int i = 0; i < 100; i++) { kde->Fill(gRandom->Gaus(2,1)); }
kde->GetValue(2); // return a nan

The code works in 6.16 ROOT version, but not 6.18