test: add non-empty coverage for TxUpdate

[mubashirrao1122]

This PR adds additional test coverage for TxUpdate::is_empty.

It verifies TxUpdate is reported as non-empty when:

• txs is populated
• seen_ats is populated
• the update is extended with a non-empty update

Tests:

• cargo test -p bdk_core --test test_tx_update

[Copilot]

Pull request overview

Adds targeted tests to ensure TxUpdate::is_empty reports false once key fields are populated or after merging in a non-empty update.

Changes:

• Add a test asserting TxUpdate becomes non-empty when txs contains a transaction.
• Add a test asserting TxUpdate becomes non-empty when seen_ats contains an entry.
• Add a test asserting TxUpdate::extend makes the receiver non-empty when extending with a non-empty update.

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