debug_strings.cls

/****************************************** 
    Imagine three strings: 
        - one with a single space 
        - one null
        - one not empty and not null
 ******************************************/     

   String singleSpaceString = ' ';
   String nullString = null;
   String notEmptyString = 'Samwise Gamgee is the real hero'; 

   // because, let's be honest...

   // Now lets concatenate these strings to see 
   // what Salesforce is doing under the hood: 

   String concatenatedText = 
      singleSpaceString + nullString + notEmptyString;

   // This is the result:

   |DEBUG|concatenatedText:  nullSamwise Gamgee is the real hero

   // What if we concatenate the empty and null variables only?

   concatenatedText = singleSpaceString + nullString; 

   // This is the result:

   |DEBUG|concatenatedText:  null