Bug 1953933 - Make TextEditor::IsEmpty() return true if it's not been initialized yet r=m_kato

`GetTextNode()` requires that the instance has already been initialized since
it requires the native anonymous `<div>`.  And `IsEmpty()` is a public method.
Therefore, it should return `true` if there is no editor root, i.e., when it's
not been initialized yet after creation or destroyed.

Additionally, `TextControlState::GetValue()` should call
`TextEditor::ComputeTextValue()` if the editor has not been initialized because
it requires the native anonymous subtree to compute the value.  I'm not sure
whether this is a bug when `<textarea>` is being initialized.
(I guess that it should get the default value of the text control element or
`mValue` in the case, but we should do that in another bug separately.)

I couldn't find the crash testcase, so, this does not contain a new test.

Differential Revision: https://phabricator.services.mozilla.com/D241543

editor/libeditor/TextEditor.cpp

@@ -480,6 +480,17 @@ already_AddRefed<Element> TextEditor::GetInputEventTargetElement() const {
 }
 
 bool TextEditor::IsEmpty() const {
+  // This is a public method.  Therefore, it might have not been initialized yet
+  // when this is called.  Let's return true in such case, but warn it because
+  // it may return different value than actual value which is stored by the
+  // text control element.
+  MOZ_ASSERT_IF(mInitSucceeded, GetRoot());
+  if (NS_WARN_IF(!GetRoot())) {
+    NS_ASSERTION(false,
+                 "Make the root caller stop doing that before initializing or "
+                 "after destroying the TextEditor");
+    return true;
+  }
   const Text* const textNode = GetTextNode();
   MOZ_DIAGNOSTIC_ASSERT_IF(textNode,
                            !Text::FromNodeOrNull(textNode->GetNextSibling()));