Send ATT_EXECUTE_WRITE_RSP when no pending writes

According to the Bluetooth Core Specification
Vol 3, Part F, 3.4.6.3 ATT_EXECUTE_WRITE_REQ,

If there are no pending prepared write values,
then no values are written,
and an ATT_EXECUTE_WRITE_RSP PDU shall be sent.

This change ensures that behavior.

Bug: 385118069
Bug: 382008796
Test: atest net_test_stack_gatt_native
Change-Id: I98142197f6276646ece034e3a288c58cf56c2c9c