Add missing [@bs.nullable] decorator

[ryyppy]

As noted by @sgrove, some decorators are missing in the index. One of them being [@bs.nullable].

Example:
https://github.com/BuckleScript/bucklescript/blob/6c6e948cc04e1615041f400080d15d3a40be7e3e/jscomp/test/return_check.ml#L63