Improve handling of empty theorems

Ensure this document will properly render (not empty theorem)

```
You can use @exm-boids to explore how these three rules influence the simulated
flocking behaviour in birds.

:::{#exm-boids}

#### Explore flocking behaviour

:::
````

Fixes #2354

src/resources/filters/crossref/theorems.lua

@@ -76,7 +76,12 @@ function theorems()
             table.insert(prefix, pandoc.Str(")"))
             table.insert(prefix, pandoc.Space())
           end
-
+
+          -- If this theorem has no content, then create a placeholder
+          if #el.content == 0 then
+            el.content[1] = pandoc.Para({})
+          end
+
           -- prepend the prefix
           local caption = el.content[1]
           tprepend(caption.content, {