support variables in optional parameter transformers

[zolamk]

Describe the bug

We currently have a set of transformer functions that provide default values to built-ins which have optional arguments the problem is with the introduction of variables in the reviewpad/backlog#158 it is not possible to pass variables to built-ins I suspect the transformer functions will start to fail when a variable is used, we need to investigate and fix the transformer functions to support variables

How to reproduce

for example doing $assignReviewer($teamMembers, 1)

Expected behavior

Transformer functions should work with variables

[zolamk]

Closed by #944