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GH-37983: [JS] create nullable Fields in Table constructor (#37982)

Previously, Tables constructed from vectors with null values would infer
Schemas that did not permit null values. This resulted in downstream
code making bad assumptions about the data. After this change, we always
assume data can be nullable, and construct a Schema with nullable
Fields.

### Are these changes tested?

I informally tested these changes for my use case, but have not tested
them more extensively

### Are there any user-facing changes?

Yes: the `Table` constructor will now produce schemas with nullable
`Fields` in situations in which it previously did not
* Closes: #37983