Support devicename=None in _sdl2.AudioDevice

The docstring already claimed that this was supported, but it didn't
work.

Author: rhendric

buildconfig/stubs/pygame/_sdl2/audio.pyi

@@ -1,4 +1,5 @@
 from collections.abc import Callable
+from typing import Optional
 
 AUDIO_U8: int
 AUDIO_S8: int
@@ -25,7 +26,7 @@ def get_audio_device_names(iscapture: bool = False) -> list[str]: ...
 class AudioDevice:
     def __init__(
         self,
-        devicename: str,
+        devicename: Optional[str],
         iscapture: bool,
         frequency: int,
         audioformat: int,
@@ -39,7 +40,7 @@ class AudioDevice:
     @property
     def deviceid(self) -> int: ...
     @property
-    def devicename(self) -> str: ...
+    def devicename(self) -> Optional[str]: ...
     @property
     def callback(self) -> Callable[[AudioDevice, memoryview], None]: ...
     @property

src_c/cython/pygame/_sdl2/audio.pyx

@@ -131,8 +131,8 @@ cdef class AudioDevice:
         memset(&self.desired, 0, sizeof(SDL_AudioSpec))
         self._iscapture = iscapture
         self._callback = callback
-        if not isinstance(devicename, str):
-            raise TypeError("devicename must be a string")
+        if devicename is not None and not isinstance(devicename, str):
+            raise TypeError("devicename must be a string or None")
         self._devicename = devicename
 
         self.desired.freq = frequency;
@@ -142,8 +142,17 @@ cdef class AudioDevice:
         self.desired.callback = <SDL_AudioCallback>recording_cb;
         self.desired.userdata = <void*>self
 
+        cdef bytes devicename_bytes
+        cdef const char* devicename_ptr
+
+        if self._devicename is None:
+            devicename_ptr = NULL
+        else:
+            devicename_bytes = self._devicename.encode("utf-8")
+            devicename_ptr = devicename_bytes
+
         self._deviceid = SDL_OpenAudioDevice(
-            self._devicename.encode("utf-8"),
+            devicename_ptr,
             self._iscapture,
             &self.desired,
             &self.obtained,