trie: small optimization of delete in fullNode case (#22979)

When deleting in fullNode, and the new child node nn is not nil, there is no need
to check the number of non-nil entries in the node. This is because the fullNode 
must've contained at least two children before deletion, so there must be another
child node other than nn.

Co-authored-by: Felix Lange <fjl@twurst.com>