Update patindex-transact-sql.md

User Feedback 366915: UUF - sql-docs-pr - PATINDEX (Transact-SQL) - SQL Server | Microsoft Learn

Author: MikeRayMSFT

docs/t-sql/functions/patindex-transact-sql.md

@@ -47,7 +47,9 @@ PATINDEX ( '%pattern%' , expression )
 **bigint** if *expression* is of the **varchar(max)** or **nvarchar(max)** data types; otherwise **int**.  
 
 ## Remarks  
-If either *pattern* or *expression* is NULL, PATINDEX returns NULL.  
+If *pattern* is NULL, PATINDEX returns NULL.
+
+If expression is NULL, PATINDEX returns error.
 
 The starting position for PATINDEX is 1.