fix(lookup): project search backend no-op

Due to an inverted condition.

modules/tools/lookup/autoload/lookup.el

@@ -237,7 +237,7 @@ This backend prefers \"just working\" over accuracy."
 
 Uses and requires `+ivy-file-search', `+helm-file-search', or `+vertico-file-search'.
 Will return nil if neither is available. These require ripgrep to be installed."
-  (unless identifier
+  (when identifier
     (let ((query (rxt-quote-pcre identifier)))
       (ignore-errors
         (cond ((modulep! :completion ivy)