MDL-29835: filemanager - zero means zero, not unlimited

The form_filemanager constructor was using empty() rather than !isset(),
so overwrote maxfiles=0 with the default (-1).  In addition, the JS UI
treated 0 as unlimited - but the non-JS fallback UI treated it as zero.
This would result in a file manager which allowed files to be
chosen/uploaded, which would then vanish into the void when the backend
saw that the maximum number of files was 0.