VariantWriter::write: Fix writing negative infinity when p_compat is true

[nikitalita]

see here: #101618 (comment)

We now default to writing -inf instead of inf_neg when writing a -infinity float; however, previous versions of the variant parser did not support reading -inf, and we still explicitly support writing variants that are compatible with previous text format versions with the p_compat flag. As such, when we write an -infinity float, we should check if the p_compat flag is set, and write inf_neg if it is.

[aaronfranke]

Makes sense to me.

[Repiteo]

Thanks!