Validate axes and led labels correctly

Before this CL, a number of checks for kl file validity were incorrect.
Some of the APIs were supposed to return an invalid value, but instead
were always returning a valid value, no matter what the input was.

Correct these values by switching to std::optional.

Bug: 266400536
Test: m libinput_tests && adb sync data && adb shell -t /data/nativetest64/libinput_tests/libinput_tests
Change-Id: I4ef45f3249dca4f4f033fb85e9fecbc2ad1f1395