Official comment
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 
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Type a default value into the fields of your choice.
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Save
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 ,'
