Use ContainerNode::hasChildren() instead of firstChild() in conditionals

Source/core/editing/InsertParagraphSeparatorCommand.cpp

Index: Source/core/editing/InsertParagraphSeparatorCommand.cpp
diff --git a/Source/core/editing/InsertParagraphSeparatorCommand.cpp b/Source/core/editing/InsertParagraphSeparatorCommand.cpp
index 1e7d049356608893a50470f869abb8f11e3363e3..f5a1d5b148503d871cd48f5db5a44bcbd4da9ace 100644
--- a/Source/core/editing/InsertParagraphSeparatorCommand.cpp
+++ b/Source/core/editing/InsertParagraphSeparatorCommand.cpp
@@ -278,7 +278,7 @@ void InsertParagraphSeparatorCommand::doApply()
             }
         } else if (isFirstInBlock && nestNewBlock) {
             // startBlock should always have children, otherwise isLastInBlock would be true and it's handled above.
-            ASSERT(startBlock->firstChild());
+            ASSERT(startBlock->hasChildren());
             refNode = startBlock->firstChild();
         }
         else if (insertionPosition.deprecatedNode() == startBlock && nestNewBlock) {