Do not use !str for checking wxString emptiness

While wxString::operator!() is a documented alternative to .empty() since
25 years, using it in just a handful of places in the codebase is not
consistent.

!str is arguably not very readable either, because std::string does not
have such an operator, and neither does e.g. Qt's QString. People would
generally except ! in the context of any kind of strings to check for null
pointer (char*), or an optional with a missing value (std::optional).

src/unix/dialup.cpp

@@ -774,7 +774,7 @@ wxDialUpManagerImpl::NetConnection wxDialUpManagerImpl::CheckPing()
         else if (wxFileExists( wxT("/usr/sbin/ping") ))
             m_PingPath = wxT("/usr/sbin/ping");
 #endif
-        if (!m_PingPath)
+        if (m_PingPath.empty())
         {
             m_CanUsePing = 0;
         }