Bug 1803044 - part 4: Make `HTMLEditor::SetAttributeOrEquivalent` insert a white-space only when old value is empty r=m_kato

This causes some new failures after applying the following patches.

Depends on D163998

Differential Revision: https://phabricator.services.mozilla.com/D163999

editor/libeditor/HTMLEditor.cpp

@@ -5818,9 +5818,12 @@ nsresult HTMLEditor::SetAttributeOrEquivalent(Element* aElement,
   if (aAttribute == nsGkAtoms::style) {
     // if it is the style attribute, just add the new value to the existing
     // style attribute's value
-    nsAutoString existingValue;
+    nsString existingValue;  // Use nsString to avoid copying the string
+                             // buffer at setting the attribute below.
     aElement->GetAttr(kNameSpaceID_None, nsGkAtoms::style, existingValue);
-    existingValue.Append(HTMLEditUtils::kSpace);
+    if (!existingValue.IsEmpty()) {
+      existingValue.Append(HTMLEditUtils::kSpace);
+    }
     existingValue.Append(aValue);
     if (aSuppressTransaction) {
       nsresult rv = aElement->SetAttr(kNameSpaceID_None, nsGkAtoms::style,