CLM SIF forward operator can lead to NaN values during assimilation testing

Describe the bug

1. List the steps someone needs to take to reproduce the bug.
   Western US test case based on Raczka et al., 2021, can lead to SIF NaN values during CLM-DART assimilation. Testing is being
   performed by @XueliHuo. Conditions that bring about NaN value are very rare (1 instance for 2 years of simulation), and SIF is
   a diagnostic variable, but a NaN can still interfere with inflation and other components of the assimilation run. The forward
   operator for SIF is external to DART and included in SourceMods based on CTSM pull request. See DART issue #338 for more
   details on original generation of SIF forward operator.

2. What was the expected outcome?
   Expected that model generated SIF to be real values.

3. What actually happened?
   Single grid cell provided a NaN value for SIF, traced back to shaded SIF calculation, which was traced back further to a slightly
   (non-physical) negative degree of light saturation that should always remain between 0-1.

Which model(s) are you working with?

Cesm2.2

Version of DART

Which version of DART are you using?
11.0.1, certainly well after latest CLM-SIF changes

Have you modified the DART code?

No -- but testing is being done with the external forward operator code, where degree of light saturation is forced between 0-1.

Build information

Please describe:

1. The machine you are running on (e.g. windows laptop, NSF NCAR supercomputer Derecho).
   CHPC at University of Utah (kingspeak/notchpeak)
2. The compiler you are using (e.g. gnu, intel).
   intel

Start with the external CLM-SIF forward operator in SourceMods and the shaded SIF calculation described in the report; review DART issue #338 for the operator's context. Reproduce the Western US test case on the stated CESM2.2/DART 11.0.1 setup and trace the rare single-cell NaN. Done means preventing the non-physical light-saturation input from producing NaN values during assimilation.