- if: '$CI_MERGE_REQUEST_SOURCE_BRANCH_NAME'

Checking for an undefined value

Examples:

    $VARIABLE == null
    $VARIABLE != null (introduced in GitLab 11.11)

It sometimes happens that you want to check whether a variable is defined or not. To do that, you can compare a variable to null keyword, like $VARIABLE == null. This expression is going to evaluate to truth if variable is not defined when == is used, or to falsey if != is used.