Whenever you create a custom field, set a default value for that if you don't want to make it a mandatory, so that it carries a default value if your contact left blank.
If you set a default value as "Friend" for the custom field "First name" then if your contact does not enter their first name then it will be recorded as "Friend".
While creating your email newsletter, if you set your custom field to be "Dear [FIRST_NAME]", if it is filled or left blank still you carry a name like "Dear Friend".
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