Fix xla_bridge_test on TPU
Fix xla_bridge_test on TPU

DETAILS:
When run xla_bridge_test on TPU v2-8 it raises the follow error about unknown backend tpu, this change set jax_platforms to be "" to eliminate this error.
```
FAILED tests/xla_bridge_test.py::GetBackendTest::test_backend_init_error - RuntimeError: Unable to initialize backend 'tpu': Unknown backend 'tpu' (set JAX_PLATFORMS='' to automatically choose an available backend)
```

TESTED:
pass unit test on both CPU and TPU
PiperOrigin-RevId: 481758573

google/jax,google/jax,google/jax,google/jax