Fixes #3031
Regression from #3000 where previously getCallerIdentifier() + callerPostfix would return an empty string.
See https://github.com/fluentassertions/fluentassertions/pull/3000/files#diff-f1890f45abb0a4df9c59f857aff667980d90f2c1ece7ea0f12e5dd85590ae8ccL105

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