Expanding empty strings is valid but pointless;
macro args already skipped doing so, now other
`beginExpansion` calls do too.
This also fixes failed interpolations (which were
already non-fatal) to continue reading characters,
not evaluate to their initial '{' character.
POSIX leaves undefined the order of output if stderr is injected into stdout,
and in practice it differs on Windows (Linux buffers both streams separately,
Windows interleaves them as they arrive without buffering).
This should help testing on other platforms
