You need to use IFNULL function to achieve that .
IFNULL(STR([F1]),'No Date Available')
Reference Article : Tableau Friction: IFNULL - is not "IF NULL", is "IF NOT NULL"
IFNULL.twbx 6.7 MB
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The simplest way is to change data type to string, then right click and select "aliases", then name the alias of Null to "No date available", then change it back to date type.
Hope it helps.