Small tidy up of code in cext.rb

Author: aardvark179

lib/truffle/truffle/cext.rb

@@ -1735,7 +1735,9 @@ def rb_class_inherited_p(ruby_module, object)
 
   def rb_get_special_vars
     vars = Primitive.cext_special_variables_from_stack
-    vars = Truffle::ThreadOperations.ruby_caller_special_variables([Truffle::CExt, Truffle::Interop.singleton_class]) if vars.nil?
+    unless vars
+      vars = Truffle::ThreadOperations.ruby_caller_special_variables([Truffle::CExt, Truffle::Interop.singleton_class])
+    end
     vars
   end