Fix chain.rb nil-safety instead of suppressing the typecheck warning

apiology asked, on the receiver_path plumbing added for castwide#1249, whether
the @sg-ignore on links.last.resolve was hiding a real bug rather than
a false positive. It wasn't reachable (Chain's constructor pads an
empty links array with UNDEFINED_CALL, so links is never empty, but
suppressing it instead of expressing that invariant in the code was
the wrong call. Extract links.last once and guard it for real.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KGu6zb5faStTC754PxMUSA
EOF
)

Author: apiology

lib/solargraph/source/chain.rb

@@ -137,9 +137,13 @@ def define api_map, name_pin, locals
             "Chain#define(links=#{links.map(&:desc)}, name_pin=#{name_pin.inspect}, locals=#{locals}) - after processing #{link.desc}, new working_pin=#{working_pin} with binder #{working_pin.binder}"
           end
         end
-        links.last.last_context = working_pin
-        # @sg-ignore Need to add nil check here
-        links.last.resolve(api_map, working_pin, locals, receiver_path)
+        # links is never empty -- the constructor pads an empty links
+        # array with UNDEFINED_CALL -- but Array#last is typed nilable.
+        last_link = links.last
+        return [] if last_link.nil?
+
+        last_link.last_context = working_pin
+        last_link.resolve(api_map, working_pin, locals, receiver_path)
       end
 
       # @param api_map [ApiMap]