feat: additional Array.prototype.split() safety

Description

follows-up on https://github.com/shapeshift/web/pull/6810 - since we can't do the noUncheckedIndexedAccess dance without having 500+ errors, this at least brings strictness for the specific case of Array.prototype.split() returning (string | undefined)[] vs. string with our current compiler options.

Note that this aims to be a non-breaking change - while the strictness is now present, all type violations have been tackled in a way that should be non-breaking, with either:

nullish coalescing to '' (i.e no different than before, since we were assuming a string)
throwing exceptions when applicable, for the specific cases where this doesn't change the flow and things would already fail as-is

Pull Request Type

[ ] :bug: Bug fix (Non-breaking Change: Fixes an issue)
[ ] :hammer_and_wrench: Chore (Non-breaking Change: Doc updates, pkg upgrades, typos, etc..)
[x] :nail_care: New Feature (Breaking/Non-breaking Change)

Issue (if applicable)

N/A, follows-up on https://github.com/shapeshift/web/pull/6810

Risk

High Risk PRs Require 2 approvals

What protocols, transaction types or contract interactions might be affected by this PR?

Low but not nil by definition - see testing steps

Testing

THOR swaps are still happy with no error thrown and with slippage still applied in the memo as before

The two above don't require new Tx - just do a full cache clear and ensure Txs look sane against develop

THOR Txs with a memo are still parsed in Tx history without errors thrown (e.g swaps)
THOR Txs without a memo are still parsed in Tx history without errors thrown (e.g sends)
Wallet creation is still happy on mobile (PR deployed to gome, just point your mobile app to gome and test it out)
Backup passphrase is still happy (test on either desktop or mobile)