Don't send events from wxTextCtrl::ChangeValue("") in wxGTK

ChangeValue() must not send events, but did in wxGTK when changing the
contents of a wxTextCtrl to be empty when it had been non-empty before.

Closes #18264.

tests/controls/textentrytest.cpp

@@ -87,6 +87,14 @@ void TextEntryTestCase::TextChangeEvents()
     entry->ChangeValue("");
     CPPUNIT_ASSERT_EQUAL( 0, updated.GetCount() );
     updated.Clear();
+
+    entry->ChangeValue("non-empty");
+    CPPUNIT_ASSERT_EQUAL( 0, updated.GetCount() );
+    updated.Clear();
+
+    entry->ChangeValue("");
+    CPPUNIT_ASSERT_EQUAL( 0, updated.GetCount() );
+    updated.Clear();
 }
 
 void TextEntryTestCase::CheckStringSelection(const char *sel)