Fix: bt_g_hash_table_contains(): handle `NULL`/0 values

Fix: bt_g_hash_table_contains(): handle `NULL`/0 values

Issue
=====
g_hash_table_lookup() returns `NULL` if the key is not found, but also
when the value is actually `NULL` (or 0). Therefore
bt_g_hash_table_contains() returns false when there's actually a
`NULL`/0 value for the given key.

Solution
========
Use g_hash_table_lookup_extended() which truly returns whether or not
the given key was found, and discard the returned value.

Known drawbacks
===============
None.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>