fix mypy failure

[dcherian]

There's a mypy failure on the scheduled CI

Found 2 errors in 1 file (checked 22 source files)
cf_xarray/accessor.py:747: note: By default the bodies of untyped functions are not checked, consider using --check-untyped-defs  [annotation-unchecked]
cf_xarray/accessor.py:793: note: By default the bodies of untyped functions are not checked, consider using --check-untyped-defs  [annotation-unchecked]
cf_xarray/accessor.py:794: note: By default the bodies of untyped functions are not checked, consider using --check-untyped-defs  [annotation-unchecked]
cf_xarray/tests/test_accessor.py:594: error: Item "None" of "_AxesBase | None" has no attribute "get_xlim"  [union-attr]
cf_xarray/tests/test_accessor.py:595: error: Item "None" of "_AxesBase | None" has no attribute "get_ylim"  [union-attr]
Error: Process completed with exit code 1.