JS API should return nulls rather than undefined when reading table data

[niloc132]

While Java arrays are initialized to nulls, GWT normalizes undefined and null to be equivalent, so we need to explicitly assign null values when building arrays.

Adds tests to ensure this doesn't break as part of #188.

Fixes #5400

[mofojed]

Looks fine, one question about the StringArray in the unit test table.
I tested in browser console with this snippet:

var dh = (await import('./dh-core.js')).default
var client = new dh.CoreClient(window.location.protocol + "//" + window.location.host);
await client.login({type: 'io.deephaven.authentication.psk.PskAuthenticationHandler', token: 'iris'});
var connection = await client.getAsIdeConnection();
var ide = await connection.startSession("python");
await ide.runCode(`
from deephaven import empty_table

x = empty_table(1).update("X=(String)null")
`)


var x = await connection.getObject({ type: 'Table', name: 'x' })
x.setViewport(0, 0)
var data = await x.getViewportData()
var value = data.get(0).get(x.columns[0])
console.log('Value is', value, ', isNull?=', value === null)