Add nullable boolean attribute to column type

[Copilot]

TAP schema had no way to describe column nullability, which is required by VOSI tables and defined in VO-DML.

Changes

• tapschema.vodsl — Added optional nullable attribute to the column otype:

  nullable: ivoa:boolean @? "indicates whether the column may have null values";
  

• tapschema.vo-dml.xml — Added corresponding column.nullable attribute element (minOccurs=0).

• tap2VOSI.xsl — Emits <flag>nullable</flag> when nullable=true; suppresses output otherwise (per VODataService spec). nullable added to the column template XPath selection.

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Original prompt

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This section details on the original issue you should resolve

<issue_title>column nullability</issue_title>
<issue_description>tapschema does not have a way of describing whether a column is nullable - this concept is required by VOSI tables and is of course described by VO-DML</issue_description>

Comments on the Issue (you are @copilot in this section)

@pahjbo A Boolean attribute should be added into the VODSL definition

• Fixes column nullability #2

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[pahjbo]

this is ready for merging

[Copilot]

Pull request overview

Adds support for representing TAP schema column nullability in the TAPSchema data model and ensures it can be emitted into VOSI Tables output, aligning the model with VO-DML and VODataService expectations.

Changes:

• Add optional nullable boolean to the column object type in tapschema.vodsl.
• Add corresponding column.nullable attribute (0..1 multiplicity) to the VO-DML XML model.
• Update tap2VOSI.xsl to emit <flag>nullable</flag> only when nullable=true.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/main/vodsl/tapschema.vodsl	Introduces optional column.nullable attribute at the VODSL model level.
src/main/vo-dml/tapschema.vo-dml.xml	Mirrors the new attribute in the VO-DML XML with minOccurs=0.
src/main/resources/tap2VOSI.xsl	Emits the VOSI nullable flag when the input column is marked nullable.

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