Adding null guard to prevent null-dereference read in EditingStyle::Init

Bug: 1038837
Change-Id: Ib53707932ff3091a427bbbc1f7bad97d469f10e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1995640Reviewed-by: Jason Chase <chasej@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Rodney Ding <rodneyding@google.com>
Cr-Commit-Position: refs/heads/master@{#732420}

third_party/blink/renderer/core/editing/editing_style.cc

@@ -510,7 +510,7 @@ void EditingStyle::Init(Node* node, PropertiesToInclude properties_to_include) {
   mutable_style_ =
       properties_to_include == kAllProperties && computed_style_at_position
           ? computed_style_at_position->CopyProperties()
-          : CopyEditingProperties(&node->GetDocument(),
+          : CopyEditingProperties(node ? &node->GetDocument() : nullptr,
                                   computed_style_at_position);
 
   if (properties_to_include == kEditingPropertiesInEffect) {