fix(ansi): skip nil children in emphasis and link renderers

[SAY-5]

Summary

Rendering markdown where an inline element (emphasis, strong, or a link) contains a child node that has no renderer crashes with a nil pointer dereference. NewElement returns an empty Element for unhandled node kinds, so its Renderer is nil, and the inline child-collection loops append that nil straight into the Children slice. EmphasisElement.doRender and LinkElement.renderTextPart then call child.Render on it and panic. The top-level renderer already guards this with if e.Renderer != nil, but the inline renderers do not. Fixes #576.

Fix

Skip nil children in EmphasisElement.doRender and LinkElement.renderTextPart before dispatching to them, matching the guard the top-level renderer already uses.

Test

TestEmphasisRenderSkipsNilChild renders an emphasis element holding a nil child. It panics at the crash site without the fix and passes with it.