[Gatt Test] Properly initialize attr value

Initialize attribute value to zero properly. Executing on x86 is filling
it with garbage value causing the test to fail.

Test: atest GattCacheTest#stored_attribute_to_binary_descriptor_test --
--abi x86
Bug: 262348171
Tag: #refactor

Change-Id: I519c116e1d5eaeff778f68c9715e54292e46b1db