refactor(dynamite): cleanup tags handling

Signed-off-by: Nikolas Rimikis <leptopoda@users.noreply.github.com>

packages/dynamite/dynamite/lib/src/openapi_builder.dart

@@ -29,24 +29,24 @@ class OpenAPIBuilder implements Builder {
         throw Exception('Only OpenAPI ${supportedVersions.join(', ')} are supported');
       }
 
-      var tags = <String?>[
-        null,
-        if (spec.paths != null) ...{
-          for (final pathItem in spec.paths!.values) ...{
-            for (final operation in pathItem.operations.values) ...{
-              ...?operation.tags,
-            },
-          },
-        },
-      ];
-      for (final tag in tags.toList()) {
-        final tagPart = tag?.split('/').first;
+      final tags = <String?>[null];
+
+      if (spec.paths != null) {
+        for (final pathItem in spec.paths!.values) {
+          for (final operation in pathItem.operations.values) {
+            if (operation.tags != null) {
+              for (final tag in operation.tags!) {
+                final tagPart = tag.split('/').first;
                 if (!tags.contains(tagPart)) {
                   tags.add(tagPart);
                 }
               }
-      tags = tags
-        ..sort(
+            }
+          }
+        }
+      }
+
+      tags.sort(
         (final a, final b) => a == null
             ? -1
             : b == null