fix(fuzz): drop a statically dead null guard in the markdown oracle (#20)

CodeQL #128 (js/comparison-between-incompatible-types): extractFrontmatter
returns an object literal on every path, so `result == null` can never be
true and the comparison is dead.

It also bought nothing. Had it ever returned null, the property reads that
follow would throw a TypeError, which the fuzzer reports as a crash exactly
the same way. The assertions that carry the contract — body is a string,
body is never longer than the input, a declined parse leaves the body
untouched — are unchanged.

Re-fuzzed after the edit: 610,641 executions, no crashes.