gh-37602: `graphs.triangulations(3)` cannot return a triangle
Fixes #37562.
As proposed in
https://github.com/sagemath/sage/issues/37562#issuecomment-1993924068,
this PR adds a doctest showing that `graphs.triangulations(3)` returns
nothing.
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URL: https://github.com/sagemath/sage/pull/37602
Reported by: David Coudert
Reviewer(s): Travis Scrimshaw
