Implement ISNULL

[leifg]

https://help.salesforce.com/apex/HTViewHelpDoc?id=customize_functions_i_z.htm&language=en_US#ISNULL

[paustint]

The Salesforce docs appear to be updated and provide some additional context about how to treat certain data types.
(although the very first sentence gives me pause "Text fields are never null"? WTF?)

https://help.salesforce.com/s/articleView?id=sf.customize_functions_isnull.htm&type=5

• Text fields are never null, so using this function with a text field always returns false. For example, the formula field IF(ISNULL(new__c) 1, 0) is always zero regardless of the value in the New field. For text fields, use the ISBLANK function instead.
• Multi-select picklist fields are never null in s-controls, buttons, and email templates, so using this function with a multi-select picklist field in those contexts always returns false.
• Empty date and date/time fields always return true when referenced in ISNULL functions.
• Don’t use ISNULL for date/time fields.
• Choose Treat blank fields as blanks for your formula when referencing a number, percent, or currency field in an ISNULL function. Choosing Treat blank fields as zeroes gives blank fields the value of zero so none of them will be null.
• Merge fields can be handled as blanks, which can affect the results of components like s-controls because they can call this function.
• When using a validation rule to ensure that a number field contains a specific value, use the ISNULL function to include fields that do not contain any value. For example, to validate that a custom field contains a value of '1', use the following validation rule to display an error if the field is blank or any other number: