fastboot: Check if super_empty.img exists before reading it.

On non-DAP devices, fastboot flash <partition> will spam error messages
about not being able to read super_empty.img. We should check that
super_empty.img exists before trying to read it.

Bug: 120429424
Test: fastboot flash system on non-DAP device
Change-Id: I6c4eec19cb3ef8d24595a75e072e1d75baaa8cdd

fastboot/fastboot.cpp

@@ -1541,7 +1541,7 @@ static bool should_flash_in_userspace(const std::string& partition_name) {
         return false;
     }
     auto path = find_item_given_name("super_empty.img");
-    if (path.empty()) {
+    if (path.empty() || access(path.c_str(), R_OK)) {
         return false;
     }
     auto metadata = android::fs_mgr::ReadFromImageFile(path);