Fix 500 on certain date input in donations#index

[ruestitch]

From bugsnag, 20250825

It looks like the bank entered an open-ended date ...

NoMethodError donations#indexundefined method 'to_fs' for nil "during the period #{start_date.to_fs(:short)} to #{end_date.to_fs(:short)}" ^^^^^^ Did you mean? to_d

It appears that this occurs when someone enters an open ended date in the picker. It also does not appear that it has any impact on the user.

[dylangriffinshub]

I'd like to take this on. Looking at DateRangeHelper#selected_interval (app/helpers/date_range_helper.rb), the existing rescue only catches Date.strptime raising. When the picker submits an open-ended range like "August 25, 2025 - " (only one side filled in), .split(" - ") produces a single-element array, so no exception is raised, but destructuring start_date, end_date = selected_interval leaves end_date as nil. That nil then hits .to_fs(:short) in selected_range_described, causing the 500. Will open a PR with a fix (falling back to the default range whenever fewer than 2 valid dates are parsed, matching the existing invalid-range behavior) plus a regression spec shortly.