Tweak process_line_done for speed

Marlin/src/gcode/queue.cpp

         SERIAL_ECHO(*p++);
     }
     SERIAL_ECHOPAIR_P(SP_P_STR, int(BLOCK_BUFFER_SIZE - planner.movesplanned() - 1));
-    SERIAL_ECHOPAIR(" B", BUFSIZE - length);
+    SERIAL_ECHOPAIR(" B", int(BUFSIZE - length));
   #endif
   SERIAL_EOL();
 }
 inline bool process_line_done(uint8_t &sis, char (&buff)[MAX_CMD_SIZE], int &ind) {
   sis = PS_NORMAL;
   buff[ind] = 0;
-  if (!ind) { thermalManager.manage_heater(); return true; }
-  ind = 0;
-  return false;
+  if (ind) { ind = 0; return false; }
+  thermalManager.manage_heater();
+  return true;
 }
 
 /**
 
       if (serial_char == '\n' || serial_char == '\r') {
 
-        if (process_line_done(serial_input_state[i], serial_line_buffer[i], serial_count[i])) continue;
+        // Reset our state, continue if the line was empty
+        if (process_line_done(serial_input_state[i], serial_line_buffer[i], serial_count[i]))
+          continue;
 
         char* command = serial_line_buffer[i];
 
-        while (*command == ' ') command++;                // Skip leading spaces
+        while (*command == ' ') command++;                   // Skip leading spaces
         char *npos = (*command == 'N') ? command : nullptr;  // Require the N parameter to start the line
 
         if (npos) {
         else if (n < 0)
           SERIAL_ERROR_MSG(MSG_SD_ERR_READ);
 
-        if (process_line_done(sd_input_state, command_buffer[index_w], sd_count)) continue;
+        // Terminate the buffer, reset the input state, continue for empty line
+        if (process_line_done(sd_input_state, command_buffer[index_w], sd_count))
+          continue;
 
         _commit_command(false);