[RareF1] Check for null inputs

[spencerp]

Patch description
Didn't catch that labels can be None.

Testing steps
Circle CI

λ pytest -k test_end2end
=================================================== test session starts ====================================================
platform linux -- Python 3.7.9, pytest-6.2.2, py-1.10.0, pluggy-1.0.0.dev0
rootdir: /private/home/spoff/ParlAI, configfile: pytest.ini, testpaths: tests, parlai/tasks
plugins: hydra-core-1.0.6, anyio-2.2.0, requests-mock-1.8.0, regressions-2.2.0, datadir-1.3.1
collected 909 items / 905 deselected / 4 selected                                                                          

tests/test_lr_schedulers.py ...                                                                                      [ 75%]
tests/nightly/gpu/test_wizard.py .                                                                                   [100%]

=================================================== slowest 10 durations ===================================================
388.53s call     tests/nightly/gpu/test_wizard.py::TestWizardModel::test_end2end
28.53s setup    tests/nightly/gpu/test_wizard.py::TestWizardModel::test_end2end
3.72s call     tests/test_lr_schedulers.py::TestLRSchedulers::test_end2end_cosine
2.07s call     tests/test_lr_schedulers.py::TestLRSchedulers::test_end2end_linear
2.07s call     tests/test_lr_schedulers.py::TestLRSchedulers::test_end2end_invsqrt

(5 durations < 0.005s hidden.  Use -vv to show these durations.)
================================ 4 passed, 905 deselected, 8 warnings in 431.16s (0:07:11) =================================