Added WebP default duration of zero when saving

[radarhere]

Resolves #6139

When saving multiple frames to a WebP image, there is currently no default duration.

Pillow/src/PIL/WebPImagePlugin.py

Line 195 in 0cf072d

duration = im.encoderinfo.get("duration", im.info.get("duration"))

But duration is added together when iterating over each frame.

Pillow/src/PIL/WebPImagePlugin.py

Line 249 in 0cf072d

timestamp = 0

Pillow/src/PIL/WebPImagePlugin.py

Lines 289 to 292 in 0cf072d

	if isinstance(duration, (list, tuple)):
	timestamp += duration[frame_idx]
	else:
	timestamp += duration

That can lead to an error, because None cannot be added to 0.

This PR adds a default duration of zero.