How can I differentiate between a field being null and being blank?

 In the context of Salesforce, it is important to differentiate between null and blank. Here is the approach given of how you can handle it in a custom validation rule by using the formula language of Salesforce:-

Checking for null

  ISBLANK(TEXT(Field__c)) && NOT(ISPICKVAL(StageName, ‘Closed Won’))

This would check if the text representation of the field is blank and it will also ensure that the ‘stageName’ is not “closed won”. It would consider both null and blank values.

Checking for blank

  ISBLANK(Field__c) && NOT(ISPICKVAL(StageName, ‘Closed Lost’))

This would help in checking if the field is blank (both null and empty string) during the time of ensuring the ‘stageName’ is not “closed Lost”.

In these examples, ISBLANK would return true if the file is null or empty. You can use ‘TEXT’ for the conversion of the field value to the text which would make it suitable for the ISBLANK function.

You can adjust the names of the fields and even conditions according to your specific requirements of the validation rule. These conditions would allow you to handle scenarios where you would need to distinguish between null and blank values in Salesforce validation rules.