Binary file arguments to flatc have to be preceded by -- to
identify them, forgetting this however results in them being
attempted to be parsed as schema/json, with cryptic errors.
This instead gives an error if 0 bytes are contained in your
text file.

Bug: 22069056
Change-Id: I226bf651dcb016f18d7c8ffadcf23466a1fc0b87
Tested: on Linux.