fix: correct Object#blank? and Object#present? polyfill implementations

Object#blank? was `!self`, ignoring objects that respond to empty?. Should
be `respond_to?(:empty?) ? !!empty? : !self` to match active_support.

Object#present? was `!!self`, which is always true for any non-nil/non-false
object regardless of blank? — inconsistent with classes that override blank?.
Should be `!blank?`.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: 2 people

lib/pact/support/core_ext.rb

@@ -36,8 +36,8 @@ def present? = !empty?
   end
 
   class Object
-    def blank? = !self
-    def present? = !!self
+    def blank? = respond_to?(:empty?) ? !!empty? : !self
+    def present? = !blank?
   end
 end