Decode empty message to nil in standard codec

[cbracken]

This updates Flutter.*Codec decode: implementations to treat empty
messages as equivalent to nil or NSNull, which avoids a crash when we
subsequently otherwise try to read the type/value from the message. This
handles the case where the sender were to send null over the channel.
e.g.,

final channel = BasicMessageChannel<Object?>('somechannel', StandardMessageCodec());
channel.send(null);

It also updates the macOS embedder to send nil to the decoder when a
zero-length message is received in order to be consistent with the iOS
embedding.

Previously, the macOS embedder always encoded platform messages as
NSData regardless of length:

engine/shell/platform/darwin/macos/framework/Source/FlutterEngine.mm

Lines 498 to 500 in ba93431

	NSData* messageData = [NSData dataWithBytesNoCopy:(void*)message->message
	length:message->message_size
	freeWhenDone:NO];

The iOS embedder did not have this issue since it special-cased
zero-length messages as nil:

engine/shell/platform/darwin/ios/framework/Source/platform_message_router.mm

Lines 23 to 26 in ba93431

	NSData* data = nil;
	if (message->hasData()) {
	data = GetNSDataFromVector(message->data());
	}

Bug: flutter/flutter#78003

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[cbracken]

I'm fine reverting the cleanup in the iOS embedder on grounds of "if it ain't broke..." but it does simplify the logic a little and removes a branch.

[stuartmorgan-g]

LGTM as is, but also changing the macOS embedding to be consistent with the iOS behavior seems reasonable.