Show fixed V in Power Display with no V sensor (#18579)

Marlin/src/inc/Conditionals_adv.h

 #if EITHER(POWER_MONITOR_CURRENT, POWER_MONITOR_VOLTAGE)
   #define HAS_POWER_MONITOR 1
 #endif
-#if ENABLED(POWER_MONITOR_VOLTAGE) && defined(POWER_MONITOR_FIXED_VOLTAGE)
+#if ENABLED(POWER_MONITOR_CURRENT) && defined(POWER_MONITOR_FIXED_VOLTAGE)
   #define HAS_POWER_MONITOR_VREF 1
 #endif
 #if BOTH(HAS_POWER_MONITOR_VREF, POWER_MONITOR_CURRENT)