Minor cleanup from lint suggestions

Author: parlough

site/lib/src/components/common/card.dart

@@ -88,7 +88,7 @@ class Card extends StatelessComponent {
       if (outlined) 'outlined-card',
       if (filled) 'filled-card',
       if (expandable) 'expandable-card',
-      if (additionalClasses != null) additionalClasses!,
+      ?additionalClasses,
     ].toClasses;
 
     final children = [

site/lib/src/components/common/client/feedback.dart

@@ -58,7 +58,7 @@ final class _FeedbackComponentState extends State<FeedbackComponent> {
               icon: 'feedback',
               title: 'Provide detailed feedback.',
               href: component.issueUrl,
-              attributes: {'target': '_blank', 'rel': 'noopener'},
+              attributes: const {'target': '_blank', 'rel': 'noopener'},
             ),
           ],
           _FeedbackState.unhelpful => [
@@ -67,7 +67,7 @@ final class _FeedbackComponentState extends State<FeedbackComponent> {
               icon: 'bug_report',
               title: 'Provide feedback or report an issue.',
               href: component.issueUrl,
-              attributes: {'target': '_blank', 'rel': 'noopener'},
+              attributes: const {'target': '_blank', 'rel': 'noopener'},
             ),
           ],
         },

site/lib/src/components/common/youtube_embed.dart

@@ -53,7 +53,7 @@ class YoutubeEmbed with CustomComponentBase {
         'videoid': videoId,
         'videotitle': videoTitle,
         'videoStartAt': '$startTime',
-        if (playlistId != null) 'playlistid': playlistId,
+        'playlistid': ?playlistId,
       },
       classes: [if (fullWidth) 'full-width'].toClasses,
       children: [

site/lib/src/extensions/code_block_processor.dart

@@ -108,7 +108,7 @@ final class CodeBlockProcessor implements PageExtension {
               content: processedContent,
               textToCopy: codeLines.copyContent,
               language: language,
-              languagesToHide: {
+              languagesToHide: const {
                 'plaintext',
                 'text',
                 'console',

site/lib/src/layouts/catalog_page_layout.dart

@@ -195,7 +195,7 @@ final class CatalogPageLayout extends DocLayout {
         ? 'Rendered example of the $name Material widget.'
         : 'Rendered image or visualization of the $name widget.';
 
-    final placeholderAlt =
+    const placeholderAlt =
         'Placeholder Flutter logo in place of '
         'missing widget image or visualization.';