efi: skip Read() calls with zero sizes

Let's avoid calling Read() with zero-sized buffer, to avoid needless firmware
quirkiness.

See: #25911

src/boot/efi/boot.c

@@ -2295,6 +2295,9 @@ static EFI_STATUS initrd_prepare(
                 if (err != EFI_SUCCESS)
                         return err;
 
+                if (info->FileSize == 0) /* Automatically skip over empty files */
+                        continue;
+
                 UINTN new_size, read_size = info->FileSize;
                 if (__builtin_add_overflow(size, read_size, &new_size))
                         return EFI_OUT_OF_RESOURCES;

src/boot/efi/util.c

@@ -326,9 +326,11 @@ EFI_STATUS file_read(EFI_FILE *dir, const char16_t *name, UINTN off, UINTN size,
         UINTN extra = size % sizeof(char16_t) + sizeof(char16_t);
 
         buf = xmalloc(size + extra);
-        err = handle->Read(handle, &size, buf);
-        if (err != EFI_SUCCESS)
-                return err;
+        if (size > 0) {
+                err = handle->Read(handle, &size, buf);
+                if (err != EFI_SUCCESS)
+                        return err;
+        }
 
         /* Note that handle->Read() changes size to reflect the actually bytes read. */
         memset(buf + size, 0, extra);