Do not register empty extension

Author: radarhere

src/PIL/SpiderImagePlugin.py

@@ -290,9 +290,9 @@ def _save(im: Image.Image, fp: IO[bytes], filename: str | bytes) -> None:
 
 def _save_spider(im: Image.Image, fp: IO[bytes], filename: str | bytes) -> None:
     # get the filename extension and register it with Image
-    filename_ext = os.path.splitext(filename)[1]
-    ext = filename_ext.decode() if isinstance(filename_ext, bytes) else filename_ext
-    Image.register_extension(SpiderImageFile.format, ext)
+    if filename_ext := os.path.splitext(filename)[1]:
+        ext = filename_ext.decode() if isinstance(filename_ext, bytes) else filename_ext
+        Image.register_extension(SpiderImageFile.format, ext)
     _save(im, fp, filename)