fix!: statistical functions should return `null` when provided a vector of only `null` values

[deephaven-internal]

This issue was auto-generated

PR: https://github.com/deephaven/deephaven-core/pull/5606 Author: lbooker42

Original PR Body

Updated Numeric functions and tests:

• avg
• absAvg
• var
• wvar
• std
• wstd
• ste
• wste
• tstat
• wtstat
• median - had incorrect behavior when null values present
• percentile - had incorrect behavior when null values present
• cov
• cor

Updated UpdateBy aggregations and tests:

• RollingAvg
• RollingStd

Updated AggBy aggregations:

• avg
• var
• std
• ReVar
• ReAvg

Will close #5564

[jjbrosnan]

I don't think this ticket warrants any actual docs work. It fixes null handling behavior of several statistical functions, and is related to tickets that widened return types.

The new null handling behavior is what users should expect to happen, and should not require any discussion in documentation.

[alexpeters1208]

I looked around in the docs and I agree - no work needed.