Clear the last of the nil checks

Guards now cover the statements that follow them, and a couple of reads
are restructured so the narrowing survives.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

exp_commands/module/commands/help.lua

@@ -71,7 +71,7 @@ Commands.new("commands", { "exp-commands_help.description" })
             page = as_number
         end
 
-        keyword = assert(keyword):lower()
+        keyword = tostring(keyword):lower()
         local pages, found
         if cache and cache.keyword == keyword then
             -- Cached value found, no search is needed