According to this page, it is currently possible to check a condition using the if smart value, however, it is not possible to set an else to this if.

UPDATE: Making some tests, this is actually possible, however, it's not documented.

Suggestion

The if function could have a possibility to set a value if the condition is not matched - something like worksheets-functions allow such as:

{{if(smartValue, "value if true", "value if false")}} 

Example

We need to filter some issues with a lookup, depending on the summary. If the summary is numeric I will check for a project and if it's not, I'll check a different project. Currently what can be done is something like the following:

statusCategory = Done {{if(issue.Summary.isNumeric(), "and project = GT")}} 

With the suggestion, we would be able to do something like:

statusCategory = Done {{if(issue.Summary.isNumeric(), "and project = GT", "and project = TEST")}} 

Workaround

As a workaround, we need to use the if/else blocks.

The default values may also be helpful if you are checking for empty issue fields, but please note these don't work for variables that were created but are empty.