To check whether a String is null or empty or blank using Apex in Salesforce, we can use the following methods

isBlank	Returns true if the specified String is white space, empty (”), or null; otherwise, returns false.
isEmpty	Returns true if the specified String is empty (”) or null; otherwise, returns false.
isNotBlank	Returns true if the specified String is not whitespace, not empty (”), and not null; otherwise, returns false.
isNotEmpty	Returns true if the specified String is not empty (”) and not null; otherwise, returns false.

Sample Code: 

        String var = ‘abc’;
        if(String.isNotBlank(var)) {
            //The string is not empty or blank or null
        }
        else {
            //The string is empty or blank or null
        }


Cheers!!!