ultrahdr: Fix nan occurence during xmp generation

The default value of hdr white nits is zero, making log content boost
to nan in certain scenarios. This can cause decode to fail on select
architectures

Bug: 285545078
Test: ./ultrahdr_enc_fuzzer

Change-Id: I72064d4f04c25d6f3c951f5c20b0b674e675d541
2 files changed