Fix(rule): PreserveInlineStyleRule assume the type of the operation data and throw stacktrace (#2028)

lib/src/models/rules/insert.dart

@@ -568,8 +568,9 @@ class PreserveInlineStylesRule extends InsertRule {
       if (prevData.endsWith('\n')) {
         /// If current line is empty get attributes from a prior line
         final currLine = itr.next();
-        final currData = currLine.data as String?;
-        if (currData != null && (currData.isEmpty || currData[0] == '\n')) {
+        final currData =
+            currLine.data is String ? currLine.data as String : null;
+        if (currData?.isEmpty == true || currData?.startsWith('\n') == true) {
           if (prevData.trimRight().isEmpty) {
             final back =
                 DeltaIterator(documentDelta).skip(index - prevData.length);