HashBuilder - Each merkle tree MUST have a root node

When an empty `hasbuilder` processes leaves **all starting with the same non-zero length prefix** a root node is NOT created while it SHOULD

Eg. with these keys starting all with same `04` nibble
```
040000[...]
040005[...]
040b00[...]
```
only a node with key `04` is created with root_hash while instead ALSO OR ONLY a root node (with empty key and `tree_mask == 0b0000000000001000`) SHOULD be created and that one should have root hash