[Test] Add test cases for MIME_HTML_ONLY with malformed multipart

[Test] Add test cases for MIME_HTML_ONLY with malformed multipart

Add tests for edge cases that caused a segfault when multipart/related
has no children or contains only non-text content:

- alternative-nested-rfc822.eml: multipart/alternative with HTML and
  related containing only image (no text), plus nested message/rfc822
- alternative-empty-related.eml: multipart/alternative with malformed
  related that has no proper MIME children

These test cases verify the NULL check fix for mp->children.