ui: cert upload: fix private key field sending empty string

ui: cert upload: fix private key field sending empty string

The private key's field is now excluded from the upload form's
JSON data if it's left empty.

Prior to this change, the form still used an empty string for the
private key's field, even if no key was provided by the user.
Because the key's field is marked as optional in the upload cert
API endpoint, JSONSchema validation would therefore fail.

Suggested-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
Signed-off-by: Max Carrara <m.carrara@proxmox.com>