Function parse_landxml_alignments_optional 

pub fn parse_landxml_alignments_optional(
    input: &[u8],
) -> Result<LandXmlAlignmentDocument, LandXmlError>

Parse a LandXML source while retaining an empty alignment collection when the document contains terrain-only content. This is for format adapters that own both source families; callers needing an alignment require the strict parse_landxml_alignments entry point above.