editor's prefill is the actual document being edited, not a placeholder
like input's — the interactive (hook-editor.ts) and RPC implementations
set it verbatim on any truthy value. Trimming before the presence check
silently dropped whitespace/blank-line prefill content from the ACP
elicitation schema, so a client-side form opened empty and accepting it
could not reproduce the extension's supplied indentation.
Now only a genuinely empty string omits `default`; any other prefill,
including whitespace-only, passes through unchanged.
Márton Danóczy
38f63b792a