fix 'Undefined index: id' errors in isFileUploadOrHasExistingValue when adding new record

I usually merge my add and edit views, so even an add view will have an id field, but it will be empty. However, if you use a separate add view, with no id field, then you'll get an "undefined index" error when using the isFileUploadOrHasExistingValue validation. This commit prevents that error.

Signed-off-by: Joshua Paling <joshua.paling@gmail.com>