When you use merge tags to personalise your campaign, it's a good idea to set default values to display for recipients who don't have any data (values) for that custom field.
Setting Fallback Values
Navigate to Settings > Custom Fields Tab
Select the Custom Field
Add the fallback value
Here is an example of a two different people receiving an SMS, in this case the Merged field was "Role Type" with a fallback value of "people"
James had a Role Type of "Marketers"
Sarah did not have a value set, so the fallback was used: