dh.ui data hook sentinel defaults are confusing

[mattrunyon]

Right now, we set empty tuples as the sentinel value for data hooks when it's a ticking table that is empty. This is not user friendly because the hook will return different data structures based on if there's data in the table or not.

Here's an example where we start w/ an empty tuple, then add data, then remove all data and are back to an empty tuple.

from deephaven import time_table, ui

_t = time_table("PT1S").update("X=ii").tail(5).where("X > 3 && X < 7")

@ui.component
def my_comp(t):
    data = ui.use_table_data(t)
    print(data)
    return t

t = my_comp(_t)

I recall there being some reason we wanted a sentinel value in these cases, but don't remember why. If we are sure we still need/want it then we should strongly consider changing the default to match the structure of the expected output. Changing the sentinel feels like an advanced feature and I'm not sure most people will care about it. They just want a consistent data structure for their data.