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/protocol/protocol.go

@@ -483,7 +483,7 @@ func (c *rawConnection) dispatcherLoop() (err error) {
 			if err := checkFilename(msg.Name); err != nil {
 				return newProtocolError(err, msgContext)
 			}
-			if msg.Size <= 0 {
+			if msg.Size < 0 {
 				return newProtocolError(fmt.Errorf("request size %d too small", msg.Size), msgContext)
 			}
 			if msg.Size > MaxRequestSize {