feat(playground): show "No output" on playgrounds that print nothing

Fixes #1594

src/theme/book.js

@@ -133,7 +133,15 @@ function playground_text(playground) {
             body: JSON.stringify(params)
         })
         .then(response => response.json())
-        .then(response => result_block.innerText = response.result)
+        .then(response => {
+            if (response.result.trim() === '') {
+                result_block.innerText = "No output";
+                result_block.classList.add("result-no-output");
+            } else {
+                result_block.innerText = response.result;
+                result_block.classList.remove("result-no-output");
+            }
+        })
         .catch(error => result_block.innerText = "Playground Communication: " + error.message);
     }
 

src/theme/css/general.css

@@ -175,3 +175,7 @@ blockquote {
     margin: 5px 0px;
     font-weight: bold;
 }
+
+.result-no-output {
+    font-style: italic;
+}