fix: traverse_bundle checks for zero trytes

[lzpap]

Solves: #291

Description

get_bundles calls traverseBundleCommand internally, which in turn calls getTrytesCommand to fetch the trytes for transactions in the bundle.

If a node doesn't have the trytes for a given transaction hash in its ledger, the value at the index of that transaction hash is either null or a string of 9s.

Therefore, getTrytesCommand doesn't return an empty list, rather a list with an empty transaction in it, meaning that all trytes of the transaction are '9's.

Detecting such cases when trying to fetch a bundle from the Tangle and throwing an exception.