[TASK] Warn when code-block has no content

resolves #821

Author: linawolf

src/RestructuredText/Directives/CodeBlockDirective.php

@@ -62,6 +62,12 @@ public function process(
         BlockContext $blockContext,
         Directive $directive,
     ): Node|null {
+        if ($blockContext->getDocumentIterator()->isEmpty()) {
+            $this->logger->warning('The code-block has no content. Did you properly indent the code? ', $blockContext->getLoggerInformation());
+
+            return null;
+        }
+
         $node = new CodeNode(
             $blockContext->getDocumentIterator()->toArray(),
         );

src/RestructuredText/Parser/LinesIterator.php

@@ -17,6 +17,7 @@
 use OutOfBoundsException;
 
 use function chr;
+use function count;
 use function explode;
 use function max;
 use function mb_strpos;
@@ -133,6 +134,11 @@ public function toArray(): array
         return $this->lines;
     }
 
+    public function isEmpty(): bool
+    {
+        return count($this->lines) === 0 || (count($this->lines) === 1 && trim($this->lines[0]) === '');
+    }
+
     /** @psalm-assert-if-false non-empty-string $line */
     public static function isEmptyLine(string|null $line): bool
     {