fix(radio): Better iteration through mixers on display, sanitize invalid sources (#3999)

* fix: mixer lines with empty source

Fixes #3367

* Fix color mixes display.

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Co-authored-by: Phil Mitchell <phil.a.mitchell@gmail.com>

radio/src/gui/gui_common.cpp

@@ -25,6 +25,7 @@
 #include "hal/adc_driver.h"
 #include "hal/switch_driver.h"
 #include "switches.h"
+#include "mixes.h"
 
 #undef CPN
 #include "MultiSubtypeDefs.h"