Stop Special:Search failing when no Elasticsearch query was run

[JeroenDeDauw]

CirrusSearch answers with an EmptySearchResultSet whenever it decides up front that nothing can match — for
instance incategory:id: with a page id that does not exist, an unknown deepcat: category, or a malformed
regex. That result set carries no Elastica result set, which the SpecialSearchResults handler dereferenced
unconditionally, so any such search on a configured wiki produced "Call to a member function getQuery() on null"
instead of a results page. The handler now records the current query only when there is one; the append handler
already renders no sidebar in that case. Tests cover both kinds of result set.

Considered, omitted: rendering the facets with empty counts as action=wbfacetsearch does, by reusing its builder
in the sidebar.

_{AI-authored — Claude Code, Opus 5 (max); one-line ask from @JeroenDeDauw covering three review findings, one
PR each; no redirections; diff not yet human-reviewed; regression tests seen failing then passing, full extension
PHPUnit suite, phpcs and phpstan clean, and the failing search reproduced and re-checked on a dev wiki.}

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.15%. Comparing base (f41af94) to head (33275af).

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #288      +/-   ##
============================================
+ Coverage     75.51%   77.15%   +1.63%     
- Complexity      429      430       +1     
============================================
  Files            50       50              
  Lines          1401     1405       +4     
============================================
+ Hits           1058     1084      +26     
+ Misses          343      321      -22     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.