@codechurn and bryanbcook are correct, but use iif condition would be easier and shorter to use:

• iif - Returns the second parameter if the first parameter evaluates to True, and the third parameter otherwise

Reference https://learn.microsoft.com/en-us/azure/devops/pipelines/process/expressions?view=azure-devops

For given question the solution could quite simple:
azure-pipelines.yml

variables: 
   namespaceName: $[ 
     iif(notIn(variables['var-a'], '', variables['noSuch']),
         $(var-a),
         $(var-b)
     )
   ]

Note: Using variables['noSuch'] as a value Null, because Null is a special literal expression that's returned from a dictionary miss, for example. Null can be the output of an expression but can't be called directly within an expression.