Conditionally send "ID" val in UploadField

The "ID" form field is not always defined,
for example in the "Replace" tab of the "versionedfiles" module
(which uses GridFieldDetailForm). Since the path to that
detail form relies on the "ID" in AssetAdmin->currentPageID(),
and does an is_numeric() check to support the "root" folder,
we need to leave the "ID" param out completely.

Checked this without versionedfiles module in admin/assets
as well as a custom UploadField in CMSMain.
Merge pull request #2630 from chillu/pulls/uploadfield-id

Conditionally send "ID" val in UploadField