native_concat: pass only strings to literal_eval

native_concat: pass only strings to literal_eval

If there is only single node and it is not a string, there is no point
in passing it into ``literal_eval``, just return it immediately.

One of the examples where passing a non-string node into
``literal_eval`` would actually cause problems is when the node is
``Undefined``. On Python 3.10 this would cause ``UndefinedError``
instead of just ``Undefined`` being returned.

Fixes #1335