Add note about NULL handling in BIT_COUNT function

[Steve Jones (way0utwest)]

Clarified behavior of BIT_COUNT with NULL values.

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[Regan Downer (v-regandowner)]

Derek Wilson (@thesqlsith)

Can you review the proposed changes?

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Pull request overview

Clarifies the documented behavior of BIT_COUNT when the input expression is NULL, ensuring readers understand the function’s null-propagation behavior.

Changes:

• Added a note stating that BIT_COUNT returns NULL when passed a NULL expression.

[learn-build-service-prod]

Learn Build status updates of commit 93195ff:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/t-sql/functions/bit-count-transact-sql.md	✅Succeeded

For more details, please refer to the build report.

[Derek Wilson (thesqlsith)]

I would suggest something like:

If expression_value is NULL of a supported data type, BIT_COUNT returns NULL. An untyped NULL literal isn't a valid argument; cast it to a supported type.
For example:

SELECT BIT_COUNT(CAST(NULL AS int));   -- Returns NULL

[Randolph West MSFT (rwestMSFT)]

Thanks Derek. I'll pull this into an internal PR.