[FIX] tools: fix TypeError in literal_eval

A TypeError is thrown when an AST node is passed to `literal_eval`
because a string is expected and the object has no len().

Check the type of the expression and make sure it's a string before
calling len() on it.

closes odoo/odoo#126551

Signed-off-by: Vranckx Florian (flvr) <flvr@odoo.com>