Only use post-card-large variation if feature_image exists

[PaulAdamDavis]

If the first post has no feature_image, it was still incorrectly displayed as a large card.

This PR adds another conditional to check feature_image exists. With this, the large variation will only be used if:

• is home page
• feature_image exists
• is the first newest post

Before:

After:

[JohnONolan]

Does this break tag archives? They depend on this code too and look (intentionally) more like the "before" screenshot, in that context

[PaulAdamDavis]

The tag archive has <header class="post-card post-card-large"> hard-coded into it, so is unaffected. This only affects posts within the loop.

Here's a screenshot of this PR in action, on a tag archive:

[JohnONolan]

aaah, awesome