feat(wallet-lib): do not sync transactions if mnemonic is absent

[jawid-h]

Issue being fixed or feature implemented

This change eliminates unnecessary synchronization of transactions in case wallet has been generated from the new mnemonic

What was done?

Check if the wallet has been generated from the new mnemonic and sync transactions from the best block (instead of block 1 or the one provided via unsafeOptions: { skipSynchronizationBeforeHeight: <n> })

How Has This Been Tested?

• Passed no mnemonic to the Wallet initialization arguments
• Ensured quick initialization time
• Hooked on account events and ensured that synchronization of the new blocks works properly

Breaking Changes

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation

For repository code-owners and collaborators only

• I have assigned this pull request to a milestone