[Form] Check $data[$name] is set when NOT dealing with a fieldset

Issue [#1988] fixes a bug when setting multiple data values from a fieldset.

This code change fixes the same problem but for the other side of that if-statement, which currently ends up in an ungraceful fatal error from clone().

([#3274] is a duplicate of this, sorry! I posted that before creating the pull request and couldn't work out how to pin the commit to it directly.)