The link to the Blog in the original question on Salesforce Interview Questions is broken, so we don't know the context. For that reason I'm going to add an update that goes beyond the answers provided.
If used in Apex (not a formula field),
isEmpty(inPutString) covers both
isBlank(inPutString) and isNull(inPutString)`. See the following references in the Apex Documentation for the String Class.
Returns true if the specified String is white space, empty (''), or null; otherwise, > returns false.
Returns true if the specified String is empty ('') or null; otherwise, returns false.
isNull() no longer applies to sets, lists, or maps. Instead,
IsEmpty() is used for sets, lists or maps with zero elements. You'll only see
isNull used in Formulas. In checking the documentation, you'll find the following:
IMPORTANT Use ISBLANK instead of ISNULL in new formulas. ISBLANK has the same functionality as ISNULL, but also supports text fields. Salesforce will continue to support ISNULL, so you do not need to change any existing formulas.