fix: removes ?? for node compat (#1576)

We shipped syntax which is beyond our `engine` constraint. `??` is
available in node 14 but 4.x is node 4 and above, while 5.x is node 12
and above.

This just dumbs it back down to `||` for now at least.

lib/chai/assertion.js

@@ -54,7 +54,7 @@ export function Assertion (obj, msg, ssfi, lockSsfi) {
   util.flag(this, 'lockSsfi', lockSsfi);
   util.flag(this, 'object', obj);
   util.flag(this, 'message', msg);
-  util.flag(this, 'eql', config.deepEqual ?? util.eql);
+  util.flag(this, 'eql', config.deepEqual || util.eql);
 
   return util.proxify(this);
 }