gatt: Improve handling outstanging encryption

When p_clcb requesting security was removed, let's keep the element on
the pending_enc_clcb but make it NULL. Later allow to handle p_clcb ==
NULL on encryption complete.

Bug: 263153991
Test: atest BluetoothInstrumentationTests
Test: manual GATT operations
Tag: #feature
Change-Id: I81f39baa45506e54f19898e7b3240967db989916