Obscure bug: External functions that call EvaluateFunction cause text to disappear

If you create an ink file with an external function that calls EvaluateFunction, it causes story content not to appear in the eventual output stream. (Apart from that, it does appear to work, so we'd best either prevent this, or support it properly!)

No file or test is named. Start by reproducing an ink story with an external function that calls EvaluateFunction, then trace why story content is missing from the eventual output stream. Done means either this combination is prevented clearly or the expected content appears in the output.

Written by the indexing model from the issue text.