Allow zero-sized files

We were rejecting files of size zero; this may have been due to a past
constraint within Permanent (and that constraint may still exist
somewhere in the backend), but we ultimately do want to let people
upload empty files since files of zero size can legitimately exist.

Issue #62 Support zero-sized files

src/validators/validateCreateFileDestinationUrlParams.ts

@@ -7,7 +7,7 @@ export const validateCreateFileDestinationUrlParams = (
   bucket: Joi.string().min(3, 'utf8').max(63, 'utf8').required(),
   fileName: Joi.string().max(1024, 'utf8'),
   fileType: Joi.string().required(),
-  maxSize: Joi.number().min(1).required(),
+  maxSize: Joi.number().min(0).required(),
   path: Joi.string(),
 }).validate(
   data,