What is the difference between null & isEmpty() in Salesforce?

ISNULL determines if an expression is null (blank) then returns TRUE if it is. If the expression contains a value, then this function returns FALSE.

isEmpty() as a condition, that generally means you're making one or even two checks too many
There are a few options to validate empty text fields in Apex: Use String. isBlank() method. This will return true if the text field is empty.