swaps: Don't select empty addresses from cache.

Boss/Mod/SwapManager.cpp

@@ -280,6 +280,8 @@ class SwapManager::Impl {
 			/* First, try to get an address from the addrcache.  */
 			auto check = tx.query(R"QRY(
 			SELECT id, address FROM "SwapManager_addrcache"
+                         WHERE address IS NOT NULL
+                          AND address <> ''
 			 ORDER BY id
 			 LIMIT 1
 			     ;