fix: extra whitespace in Castform header

[busches]

Using the Castform template without a summary, still has extra whitespace for the summary. This commit fixes the extra whitespace to not appear when no summary is present.

Closes #1194

Before:

After:

[AmruthPillai]

Good catch, thank you for the contribution! :)

[busches]

@AmruthPillai when you do think you'll release this?

[e] I was able to work around it with this custom css for now:

.Castform_main__2wJo1 > div > div {
    display: none;
}