Always replace the placeholder even if it's empty

And keep the placeholder text if we don't know what it is. This way if we see the placeholder was not replaced, we know that it must be a typo.

This is raised when we tried to use {{labels}} for issues without any labels. The end result was still having {{labels}}, and I was puzzled why it didn't work, only to figure out that because it's empty. If I saw it replaced by an empty string, I'll immediately understand that there's no labels.

Context: gitlab-org/quality/triage-ops!38 (comment 109197063)