gui: Disable unavailable context menu items in transactions tab

Summary:
Disable "Copy label" and "Copy address" context menu actions when no label or address is available for a transaction.

This is a backport of core#17956

Test Plan:
ninja && src/qt/bitcoin-qt

Right click on a transaction with no label.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9925