Set sample from context when absent in workspace

Fix #385.

Signed-off-by: Don Naegely <naegelyd@gmail.com>
Those recent changes [always set the value as a list in the context](https://github.com/cbmi/varify/blob/9094abbe86fc2b6260720711baa66c6e124b7ab7/varify/static/js/src/ui/modals/sample.js#L359-L364). I suppose someone who hasn't logged in for quite some time might have a "legacy" context that would not be in list form. I will update this to make sure the returned value is a list.