OpenALBindings: fix crash on HL when passing NULL device

[ACrazyTown]

The following code produces an access violation when targeting Hashlink:

import lime.media.openal.ALC;

class Main extends lime.app.Application
{
	public function new()
	{
		super();
		trace(ALC.getString(null, ALC.ALL_DEVICES_SPECIFIER));
	}
}

The OpenAL specification mentions that passing in null to ALC.getString() is allowed in certain cases, so we should handle null gracefully instead of blindly trying to read the pointer.

(...)
Specifying NULL for deviceHandle when asking for ALC_EXTENSIONS will generate
an ALC_INVALID_DEVICE error. The deviceHandle value is ignored when asking for
ALC_DEFAULT_DEVICE_SPECIFIER or
ALC_CAPTURE_DEFAULT_DEVICE_SPECIFIER.
(...)
An alcGetString query of ALC_DEVICE_SPECIFIER or
ALC_CAPTURE_DEVICE_SPECIFIER with a NULL device passed in will return a list
of available devices. Each device name will be separated by a single NULL character and
the list will be terminated with two NULL characters.

[joshtynjala]

Thank you!