Fix panic on empty mapping value at EOF

[lepton9]

This fixes a crash when parsing an entry with an empty value at end-of-file.

The parser consumes the .eof token while returning .none, leaving token_it.pos past the end of the token buffer. Later, in Parser.map() the Parser.getCol() causes the OOB access.

This should be valid YAML and pass:

test "empty map value at eof" {
    try parseSuccess(
        \\key:
        \\
    );
}