Fix EEPROM errors when EXTRUDER == 0

Partly my fault from last time (MarlinFirmware#16464)
Problem appears to be that forcing store/retrieve of one element is inconsistent with offsetof() calculation based on size of planner_extruder_advance_K which is zero.  Instead, use save/retrieve of zero elements, which is natural for the saving (remove _MAX()) and for the retrieval a simple #ifdef covers it.

Marlin/src/module/configuration_store.cpp

@@ -1246,7 +1246,7 @@ void MarlinSettings::postprocess() {
         EEPROM_WRITE(planner.extruder_advance_K);
       #else
         dummy = 0;
-        for (uint8_t q = _MAX(EXTRUDERS, 1); q--;) EEPROM_WRITE(dummy);
+        for (uint8_t q = EXTRUDERS; q--;) EEPROM_WRITE(dummy);
       #endif
     }
 
@@ -2091,12 +2091,14 @@ void MarlinSettings::postprocess() {
       // Linear Advance
       //
       {
-        float extruder_advance_K[_MAX(EXTRUDERS, 1)];
         _FIELD_TEST(planner_extruder_advance_K);
-        EEPROM_READ(extruder_advance_K);
-        #if ENABLED(LIN_ADVANCE)
-          if (!validating)
-            COPY(planner.extruder_advance_K, extruder_advance_K);
+        #if EXTRUDERS
+          float extruder_advance_K[EXTRUDERS];
+          EEPROM_READ(extruder_advance_K);
+          #if ENABLED(LIN_ADVANCE)
+            if (!validating)
+              COPY(planner.extruder_advance_K, extruder_advance_K);
+          #endif
         #endif
       }