WatchServer: Ignore files with size = 0

[SBriere]

File with size = 0 should be ignored and put in the "Rejected" folder, since they seem to cause issue with SFTP transfer.

[AntoineGuillerand]

The file seems to be considered as transfered from the watch and will be deleted if the option is checked.
Maybe add a verification on the file size before acknowledging the transfer as done.

[SBriere]

The issue doesn't seem to affect the SFTP transfer in itself (as I tested with 0 byte files without any issue), but might be occurring when the watch transfers its data. I therefore added some checks on file size and replied with an error to the watch if the file size doesn't match the expected content-length or is 0 in size.