Defrag should only skip non-existent dbs, not empty dbs

In valkey-io#1609, we now doing on-demand database allocation instead
of preallocation. And in beginDefragCycle, we should not skip
empty databases if they exist. The defrag still defrags the
internal allocations of the hashtables structs if they exist.

Call chain: defragStageDbKeys -> defragStageKvstoreHelper ->
kvstoreHashtableDefragTables -> hashtableDefragTables.

Signed-off-by: Binbin <[email protected]>

Author: enjoy-binbin

src/defrag.c

@@ -941,6 +941,7 @@ static doneStatus defragStageDbKeys(monotime endtime, void *target, void *privda
     UNUSED(privdata);
     int dbid = (uintptr_t)target;
     serverDb *db = server.db[dbid];
+    if (db == NULL) return DEFRAG_DONE;
 
     static defragKeysCtx ctx; // STATIC - this persists
     if (endtime == 0) {
@@ -959,6 +960,7 @@ static doneStatus defragStageExpiresKvstore(monotime endtime, void *target, void
     UNUSED(privdata);
     int dbid = (uintptr_t)target;
     serverDb *db = server.db[dbid];
+    if (db == NULL) return DEFRAG_DONE;
     return defragStageKvstoreHelper(endtime, db->expires,
                                     scanHashtableCallbackCountScanned, NULL, NULL);
 }
@@ -1227,7 +1229,6 @@ static void beginDefragCycle(void) {
     defrag.remaining_stages = listCreate();
 
     for (int dbid = 0; dbid < server.dbnum; dbid++) {
-        if (dbHasNoKeys(dbid)) continue;
         addDefragStage(defragStageDbKeys, (void *)(uintptr_t)dbid, NULL);
         addDefragStage(defragStageExpiresKvstore, (void *)(uintptr_t)dbid, NULL);
     }