RobotFramework: empty variable check suddenly not working anymore - robotframework

Question

I'm playing around with command-line arguments and had something working both with / without giving a command-line argument, but suddenly it starts failing if I don't give a command-line argument.
On https://groups.google.com/g/robotframework-users/c/_5Usp-K4Dlw?pli=1 I read that adding a space before the variable and comparing it to a value like this '${ VAR}'==<value> should work to check if a variable is existing or not.
The code-snippet that was working before is:
*** Test Cases ***
My test
[Documentation] test to check if the SUT reacts as expected
${is_test}= Evaluate '${ VAR}'=='test'
Log To Console ${is_test}
Run Keyword If ${is_test} Log To Console VAR == test
After changing the code to (Removed '[Documentation]' since it was copied from another test):
*** Test Cases ***
My test
${is_test}= Evaluate '${ VAR}'=='test'
Log To Console ${is_test}
Run Keyword If ${is_test}
... Log To Console VAR == test
it suddenly started failing with: Variable '${ VAR}' not found. errors.
And after changing it back to the original it still fails.
I can't figure what I did wrong in the change.
Note: The company I'm working for uses RobotFramework version 3.0.4 (yes I know ancient)

Answer 1

Managed to solve it by adding ${VAR} with some default value to a *** Variables *** list. But still I don't understand why it was originally working without that list.

Answer 2

Here is how your test should look like if you want to check if variable is empty and do something based on that.
*** Variables ***
${VAR}= test
*** Test Cases ***
My test
Run Keyword If '${VAR}'=='${EMPTY}' Log To Console Variable is empty
... ELSE Log To Console Variable is not empty
If you want variable default value to be empty and execute some keyword only if it comes from command line set variable as empty, for example:
*** Variables ***
${VAR}= ${EMPTY}
*** Test Cases ***
My test
Run Keyword If '${VAR}'=='${EMPTY}' Log To Console Variable is not set
... ELSE Log To Console Variable is ${VAR}
Run your test from command-line without passing variable
$ robot sample.robot
==============================================================================
Sample
==============================================================================
My test Variable is not set
My test | PASS |
------------------------------------------------------------------------------
Sample | PASS |
1 test, 1 passed, 0 failed
==============================================================================
Now, run with passing variable
$ robot -v VAR:test sample.robot
==============================================================================
Sample
==============================================================================
My test Variable is test
My test | PASS |
------------------------------------------------------------------------------
Sample | PASS |
1 test, 1 passed, 0 failed
==============================================================================
P.S. About space inside variable, I think it's just typo in Google Groups. If it was working then variable might be globally accessible or defined somewhere else.