The merge field will be be blank when the message is sent to any contacts who don't have anything in that field of their contact record unless you've setup a default merge field value.
A default merge field value will substitute a word (or words) of your choosing in where ever a contact who has no content in that field.
For example, a common default merge field for [First Name] is friend.
To set a default merge field: 1. Go to Contacts > Settings > Default Merge Fields
Type a default value into the fields of your choice.
Example: Say you have a contact’s email address but not their first name stored in their Contact Record. Were you to send them an email from your account containing the [First Name] Merge Field (which would normally populate with the first name from that contact’s record unless that field is blank), you could set up a default Merge Field to have that field populate with something like ‘Friend’ so that their email will read something like ‘Hi Friend, ’ instead of 'Hi ,'