sql: expect zero-column results from PRAGMA incremental_vacuum

The fact that `PRAGMA incremental_vacuum` may return a zero-column
SQLITE_ROW result is documented in `sqlite3_data_count()` documentation:
<https://www.sqlite.org/c3ref/data_count.html>

Previously successful auto_vacuum worked,
but resulted in a "Failed to run incremental vacuum" log.

CHANGELOG.md

@@ -14,6 +14,7 @@
   or `dc_jsonrpc_instance_t` is unreferenced
   during handling the JSON-RPC request. #4153
 - Delete expired messages using multiple SQL requests. #4158
+- Do not emit "Failed to run incremental vacuum" warnings on success. #4160
 
 
 ## 1.111.0

src/sql.rs

@@ -713,12 +713,22 @@ pub async fn housekeeping(context: &Context) -> Result<()> {
 
     // Try to clear the freelist to free some space on the disk. This
     // only works if auto_vacuum is enabled.
-    if let Err(err) = context
+    match context
         .sql
-        .execute("PRAGMA incremental_vacuum", paramsv![])
+        .query_row_optional("PRAGMA incremental_vacuum", (), |_row| Ok(()))
         .await
     {
-        warn!(context, "Failed to run incremental vacuum: {}", err);
+        Err(err) => {
+            warn!(context, "Failed to run incremental vacuum: {err:#}");
+        }
+        Ok(Some(())) => {
+            // Incremental vacuum returns a zero-column result if it did anything.
+            info!(context, "Successfully ran incremental vacuum.");
+        }
+        Ok(None) => {
+            // Incremental vacuum returned `SQLITE_DONE` immediately,
+            // there were no pages to remove.
+        }
     }
 
     if let Err(e) = context