fix: write GML without explicit vertex IDs

[LeonidasZhak]

Summary

Allow write_graph(..., format = "gml") to use the GML writer's default vertex IDs when id is omitted. Fixes #2710.

Thanks to maintainers

Thanks for triaging the report and asking for a regression test.

Issue or motivation

Writing any graph as GML currently fails unless the caller supplies one ID per vertex. Earlier versions generated node IDs when none were supplied.

Root cause

Stimulus treated the C writer's nullable id pointer as a required vector. The generated R wrapper converted NULL to numeric(0), so C received an empty vector instead of a null pointer and rejected its length.

Change

Mark id as optional in the Stimulus metadata and regenerate both interface layers. The added test writes a graph without IDs and reads it back to confirm graph isomorphism.

Tests

• air format .
• Focused foreign-format tests: 13 passed, 0 failed, 0 warnings
• devtools::check() with CRAN lifecycle 1.0.5: all 8,185 package tests passed; 0 errors, with three unchanged local warnings from vendored GLPK and unavailable checkbashisms

Scope

Explicit IDs, other file formats, GML serialization details, and vendored C sources are unchanged.

I used AI assistance to help check for issues, refine the code, and run tests.

I'm fine with GPL 2 or later and FDL.

• By submitting this pull request, I assign the copyright of my contribution to The igraph development team.