Avoid busy wait in get_slices thread by using the right check

This reduces the CPU usage of Allen in production by 1 core.

AllenOnline/src/MEPProvider.cpp

@@ -116,7 +116,7 @@ std::tuple<bool, bool, bool, size_t, size_t, std::any> MEPProvider::get_slice( s
   if ( !m_read_error ) {
     // If no transposed slices are ready for processing, wait until
     // one is; use a timeout if requested
-    if ( m_transposed.empty() ) {
+    if ( !has_transposed() ) {
       auto wakeup = [this, &has_transposed] { return ( has_transposed() || m_read_error || m_transpose_done ); };
       if ( timeout ) {
         timed_out = !m_transposed_cond.wait_for( lock, std::chrono::milliseconds{*timeout}, wakeup );