explorer: Invalid public account address throws 500 error on TX fetch

Describe the bug
When entering an invalid BLS public account address on the explorer, the balance component handles it correctly and displays "N/A", indicating the account doesn't exist or is malformed. However, the transaction retrieval still proceeds and results in a HTTP 500 error.

This happens because the frontend still attempts to fetch transactions for an invalid account.

To Reproduce

1. Open the account page on the explorer by using a malformed BLS key.
2. Observe the 500 error being propagated to the web page in the transaction overview, and the balance shown correctly as N/A.

Expected behaviour
If the input is not a valid BLS public key we should:

1. Not attempt to fetch transactions
2. Show a clear message like: "Invalid Public Account Address"

Logs/Screenshot

Start at the explorer account page using the malformed BLS key from the reproduction steps and trace the transaction request separately from the balance handling. Confirm that invalid input avoids the transaction fetch, displays an "Invalid Public Account Address" message, and no longer propagates a 500 error.