What is output when we write null = null in where clause

What is the output of following query.

Query 1: Select * from sysobjects where null is null

Output
It return the total number of records of sysobjects.
1

Query 2: Select * from sysobjects where null = null

Output
It return 0 records of sysobjects.

Query 3: Select * from sysobjects where isnull(null,'')=''

Output
It return the total number of records of sysobjects.
1

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