SET @ADR3 = 'null'

That's setting the variable to the string value n-u-l-l. If you want to check whether a variable has the string value "null", then you'd need IF @Adr3 = "Null"

If you want to set and check for the absence of a value (NULL), then it's

SET @Adr3 = NULL

IF @Adr3 IS NULL ...