Ensure left search results panel is always visible.

[mbien]

initialize divider position with a minimal value. This will always reset the value to at least 250 even if it is changed by the user to less.

fixes #8064

I could reproduce this sometimes with

• fresh user conf (no divider location stored)
• search something in an empty folder (e.g favorites view; ensures that the search finishes fast with no results)

I believe what happens in this case is that the divider isn't initialized (which is fine by itself) but the computed width of the left panel is 0 for some reason too (which is not ok).