core, ethdb: two tiny fixes (#17183)

* ethdb: fix memory database

* core: fix bloombits checking

* core: minor polish

ethdb/database_test.go

@@ -59,6 +59,28 @@ func TestMemoryDB_PutGet(t *testing.T) {
 func testPutGet(db ethdb.Database, t *testing.T) {
 	t.Parallel()
 
+	for _, k := range test_values {
+		err := db.Put([]byte(k), nil)
+		if err != nil {
+			t.Fatalf("put failed: %v", err)
+		}
+	}
+
+	for _, k := range test_values {
+		data, err := db.Get([]byte(k))
+		if err != nil {
+			t.Fatalf("get failed: %v", err)
+		}
+		if len(data) != 0 {
+			t.Fatalf("get returned wrong result, got %q expected nil", string(data))
+		}
+	}
+
+	_, err := db.Get([]byte("non-exist-key"))
+	if err == nil {
+		t.Fatalf("expect to return a not found error")
+	}
+
 	for _, v := range test_values {
 		err := db.Put([]byte(v), []byte(v))
 		if err != nil {