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_test.go

@@ -891,7 +891,7 @@ func TestPullCtxCancel(t *testing.T) {
 	emptyState := func() pullBlockState {
 		return pullBlockState{
 			sharedPullerState: newSharedPullerState(protocol.FileInfo{}, nil, f.folderID, "", nil, nil, false, false, protocol.FileInfo{}, false, false),
-			block:             protocol.BlockInfo{},
+			block:             protocol.BlockInfo{Size: 42},
 		}
 	}