check if ext params is not empty

Author: bonneted

deepxde/model.py

@@ -403,7 +403,7 @@ def _compile_jax(self, lr, loss_fn, decay):
             self.net.params = self.net.init(key, self.data.test()[0])
         external_trainable_variables_val = [
             var.value for var in self.external_trainable_variables
-        ]
+        ] if self.external_trainable_variables else []
         self.params = [self.net.params, external_trainable_variables_val]
         # TODO: learning rate decay
         self.opt = optimizers.get(self.opt_name, learning_rate=lr)