Merge #13265: wallet: Exit SyncMetaData if there are no transactions to sync

Summary:
b0d2ca9fb66d793e3c0f2e6ede811f1b16c33a9f wallet: Exit SyncMetaData if there are no transactions to sync (Wladimir J. van der Laan)

Pull request description:

Instead of crash with an assertion error, simply exit the function `SyncMetaData` if there is no metadata to sync.

Fixes #13110.

Tree-SHA512: 44c4789497b5b63963bef66d8b695987dde80764199f6ea0f2c974be19d29c2663f32446a663a2ee9029e143e5d1d9e8a591e52e6e7e795b982782626bec25bb

Backport of Core PR13265

This skips PR11074 and should avoid the issue found here: https://github.com/bitcoin/bitcoin/issues/13110/

Test Plan:

ninja check
./bitcoin-qt

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5435