Don't clear Preview when Response Filter is invalid/non-existent

Following on from #1258:

Short version: Only update the Response Preview when the Response Filter evaluation returns something worth showing.

In more detail:

• *Given:* the Preview pane is displaying any response content
• *When:* the Response Filter input changes
• *And:* the Response Filter input evaluates as either syntactically invalid or non-existent in the current document
• *Then:* don't update the Preview to show [] or similar, as it hides the document tree that the user is currently trying to navigate.

Example: If I'm trying to filter down to $.store.books[*].author, then as soon as I type $. the Preview just shows [], and I won't see anything useful until I either delete or type as far as $.store . This is particularly annoying if I'm continually looking at the Preview as a guide to the path I'm typing.