Fix crash from a missing check for an environment variable on macOS

Ref: !136472

source/blender/windowmanager/intern/wm_splash_screen.cc

@@ -365,7 +365,8 @@ static uiBlock *wm_block_splash_create(bContext *C, ARegion *region, void * /*ar
 #  if defined(__APPLE__)
   if (is_using_macos_rosetta() > 0)
 #  elif defined(_M_X64)
-  if (strncmp(BLI_getenv("PROCESSOR_IDENTIFIER"), "ARM", 3) == 0)
+  const char *proc_id = BLI_getenv("PROCESSOR_IDENTIFIER");
+  if (proc_id && strncmp(proc_id, "ARM", 3) == 0)
 #  endif
   {
     uiItemS_ex(layout, 2.0f, LayoutSeparatorType::Line);