Return empty string instead of nil when linting on required (#4748)

* Return empty string instead of nil when linting on required

This allows lint to work in scenarios when required is used in secrets or it's output is passed to another function.
Due to lint mode no longer failing on missing value in required it is passing nil through which not all functions can accept.

Fixes #4747

Signed-off-by: Helgi Þorbjörnsson <helgith@gmail.com>

* Apply suggestions from code review

Co-Authored-By: helgi <70530+helgi@users.noreply.github.com>
Signed-off-by: Helgi Þorbjörnsson <helgith@gmail.com>

* Add tests

Signed-off-by: Helgi Þorbjörnsson <helgith@gmail.com>
(cherry picked from commit 15703cb1)