Fix error message when the uploaded file is empty

Steps to reproduce:

1. go to a workspace where you can create a File document
2. click on the New button then on File
3. in the creation form, fill the different field and choose an empty file for the content (created by touch foo.txt under Linux)
4. push the Create button
5. a validation message is displayed : "Invalid file"

This message corresponds to the label

error.inputFile.invalidFile=Invalid file

whereas we should use

error.inputFile.emptyFile=Empty file