No explicit handling of nans in lookup table enhancement

[mraspaud]

Describe the bug
While working on #2933, I saw there was a warning about coercing floats to ints in the lookup table enhancement test.
Turns out that the test uses data that contains NaNs, which don't seem to be explicitly taken care of.

To Reproduce

pytest -s satpy/tests/enhancement_tests/test_enhancements.py::TestEnhancementStretch::test_lookup

Expected behavior
I expect the test not to raise an warning, and that there was a way to explicitly tell the enhancement what to do with NaNs

Actual results
A warning is issued:

satpy/tests/enhancement_tests/test_enhancements.py::TestEnhancementStretch::test_lookup
  /home/a001673/usr/src/satpy/satpy/enhancements/__init__.py:292: RuntimeWarning: invalid value encountered in cast
    band_data = band_data.clip(0, lut.size - 1).astype(np.uint8)

Environment Info:

• OS: Linux
• Satpy Version: main