fix(protocol): handle zero-size requests (fixes #10709) (#10710)

- Allow zero-sized requests since they are sent by all current versions
of Syncthing.
- Stop sending zero-sized requests since that's stupid.

---------

Signed-off-by: Jakob Borg <jakob@kastelo.net>

lib/model/folder_sendrecv.go

@@ -1345,6 +1345,12 @@ func (f *sendReceiveFolder) copierRoutine(ctx context.Context, in <-chan copyBlo
 			default:
 			}
 
+			if block.Size == 0 {
+				// Copying zero bytes is a no-op.
+				state.copyDone(block)
+				continue
+			}
+
 			if !f.DisableSparseFiles && state.reused == 0 && block.IsEmpty() {
 				// The block is a block of all zeroes, and we are not reusing
 				// a temp file, so there is no need to do anything with it.
@@ -1534,6 +1540,13 @@ func (f *sendReceiveFolder) pullerRoutine(ctx context.Context, in <-chan pullBlo
 
 		bytes := state.block.Size
 
+		if bytes == 0 {
+			// Pulling zero bytes is a no-op.
+			state.pullDone(state.block)
+			out <- state.sharedPullerState
+			continue
+		}
+
 		if err := requestLimiter.TakeWithContext(ctx, bytes); err != nil {
 			state.fail(err)
 			out <- state.sharedPullerState