fix: guard against null fields in Card external-link branch

The condition '!fields && !fields.slug' would throw a TypeError when
fields is null/undefined because the second operand still accesses
.slug on the falsy value. Use '(!fields || !fields.slug)' instead.

Signed-off-by: Lee Calcote <lee.calcote@layer5.io>

Author: leecalcote

src/components/Card/index.js

@@ -71,7 +71,7 @@ const Card = ({
                 see more <IoIosArrowRoundForward />
               </Link>
             )}
-            {!fields && !fields.slug && frontmatter.eurl && (
+            {(!fields || !fields.slug) && frontmatter.eurl && (
               <a
                 className="external-link-btn"
                 href={frontmatter.eurl}