fix avg aggregation

- ignore NULL rows as SQLite do
- emit NULL instead of NaN when no rows were aggregated
- adjust agg column alias name

bindings/javascript/packages/native/promise.test.ts

@@ -72,6 +72,37 @@ test('explicit connect', async () => {
     expect(await db.prepare("SELECT 1 as x").all()).toEqual([{ x: 1 }]);
 })
 
+test('avg-bug', async () => {
+    const db = await connect(':memory:');
+    const create = db.prepare(`create table "aggregate_table" (
+        "id" integer primary key autoincrement not null,
+        "name" text not null,
+        "a" integer,
+        "b" integer,
+        "c" integer,
+        "null_only" integer
+    );`);
+
+    await create.run();
+    const insert = db.prepare(
+        `insert into "aggregate_table" ("id", "name", "a", "b", "c", "null_only") values (null, ?, ?, ?, ?, null), (null, ?, ?, ?, ?, null), (null, ?, ?, ?, ?, null), (null, ?, ?, ?, ?, null), (null, ?, ?, ?, ?, null), (null, ?, ?, ?, ?, null), (null, ?, ?, ?, ?, null);`,
+    );
+
+    await insert.run(
+        'value 1', 5, 10, 20,
+        'value 1', 5, 20, 30,
+        'value 2', 10, 50, 60,
+        'value 3', 20, 20, null,
+        'value 4', null, 90, 120,
+        'value 5', 80, 10, null,
+        'value 6', null, null, 150,
+    );
+
+    expect(await db.prepare(`select avg("a") from "aggregate_table";`).get()).toEqual({ 'avg ("a")': 24 });
+    expect(await db.prepare(`select avg("null_only") from "aggregate_table";`).get()).toEqual({ 'avg ("null_only")': null });
+    expect(await db.prepare(`select avg(distinct "b") from "aggregate_table";`).get()).toEqual({ 'avg (DISTINCT "b")': 42.5 });
+})
+
 test('on-disk db', async () => {
     const path = `test-${(Math.random() * 10000) | 0}.db`;
     try {