libsecret: fix warning with move to net6.0 TFM

Moving to target .NET 6 directly with the core library has unearthed a
bug/warning in the secret service implementation. IntPtr is a value
type, so it can never be null.. we should have been comparing this
pointer to `IntPtr.Zero`.

Author: mjcheetham

src/shared/Core/Interop/Linux/SecretServiceCollection.cs

@@ -65,7 +65,7 @@ public unsafe ICredential Get(string service, string account)
                     throw new InteropException("Failed to search for credentials", code, new Exception(message));
                 }
 
-                if (results != null && results->data != null)
+                if (results != null && results->data != IntPtr.Zero)
                 {
                     SecretItem* item = (SecretItem*) results->data;