partially revert static outputString in RenderCurrentFolder

The optimization only applies to targets with DGUS2_TFT set. On other
targets the fixed-size array may be of insufficient size.

However, remove the potentially problematic initialization with a single
nullpointer (empty string), as this will be overwritten in the very next
line anyway.

Fixes: a9c018f18c

Marlin/src/lcd/extui/knutwurst/anycubic_touchscreen.cpp

@@ -885,10 +885,12 @@ void AnycubicTouchscreenClass::RenderCurrentFolder(uint16_t selectedNumber) {
         fileNameWasCut = true;
         fileNameLen    = MAX_PRINTABLE_FILENAME_LEN;
       }
+      static char outputString[MAX_PRINTABLE_FILENAME_LEN];
+  #else
+      char outputString[fileNameLen];
   #endif
 
       // Bugfix for non-printable special characters which are now replaced by underscores.
-      static char outputString[MAX_PRINTABLE_FILENAME_LEN] = {'\0'};
       for (unsigned int i = 0; i <= fileNameLen; i++) {
         if (isPrintable(fileName[i])) {
           outputString[i] = fileName[i];