When no sources are selected, query-index dropdown allows all selections

When the Source filter is empty (as is the initial state when the Codex Search app starts up), all sources are used since no selection has been made. Therefore the query-index dropdown should omit those indexes that are not supported by the EBSCO KB Codex source.

This query-index restriction works correctly when both sources are selected (or the EBSCO KB alone is selected) – just not when there is no selection.

This is a regression.