Fix a crash when using --no-generate-docs (#2137)

* Fix a crash when using --no-generate-docs

* Move check to _parseDocumentation

Author: jdkoren

lib/src/model/documentation.dart

@@ -65,6 +65,9 @@ class Documentation {
 
   /// Returns a tuple of List<md.Node> and hasExtendedDocs
   Tuple2<List<md.Node>, bool> _parseDocumentation(bool processFullDocs) {
+    if (!_element.hasDocumentation) {
+      return Tuple2([], false);
+    }
     String text = _element.documentation;
     showWarningsForGenericsOutsideSquareBracketsBlocks(text, _element);
     MarkdownDocument document =

lib/src/render/documentation_renderer.dart

@@ -13,6 +13,9 @@ abstract class DocumentationRenderer {
 class DocumentationRendererHtml extends DocumentationRenderer {
   @override
   Tuple2<String, String> render(List<md.Node> nodes, bool processFullDocs) {
+    if (nodes.isEmpty) {
+      return Tuple2('', '');
+    }
     var rawHtml = md.HtmlRenderer().render(nodes);
     var asHtmlDocument = parse(rawHtml);
     for (var s in asHtmlDocument.querySelectorAll('script')) {