When the number of documents in the window frame is 0, the adjusted count for $stdDevSamp might be negative, and we should return NULL in this case.

https://github.com/10gen/mongo/blob/fc174d081bdd6a5178cc888e6f0309d23504b4df/src/mongo/db/pipeline/window_function/window_function_stddev.h#L65-L67

$covarianceSamp handles this correctly.

https://github.com/10gen/mongo/blob/fc174d081bdd6a5178cc888e6f0309d23504b4df/src/mongo/db/pipeline/window_function/window_function_covariance.cpp#L96-L98