fix(ansi): skip nil inline child renderers

[syf2211]

Summary

Guard emphasis, link, and table cell renderers against nil children when Goldmark inline nodes are unhandled by NewElement. This prevents panics when custom or extension inline nodes appear inside inline structures.

Fixes #576

Motivation

When Glamour encounters unhandled or custom inline nodes (e.g. from Goldmark parser extensions), NewElement falls through to the default case and returns an Element with a nil Renderer. That nil value is appended to Children slices for emphasis, links, and table cells. Calling child.Render() on these nil interface values causes a nil pointer dereference panic.

Changes

• Add nil checks in EmphasisElement.doRender, LinkElement.renderTextPart, and TableCellElement.Render before invoking child renderers
• Add unit tests covering all three element types with a nil child followed by a valid child

Tests

• go test ./ansi/ -run 'Test.*SkipsNilChildren' -v -count=1 — PASS
• go test ./... -count=1 — PASS

Notes

• Unhandled nodes still log the existing Warning: unhandled element message and produce no output; this change only prevents the panic.
• Pre-PR review: composer-2.5 reviewed — APPROVE.