ClockFilter NPEs on Empty Tables

[cpwright]

Description

A clear and concise description of what the bug is.

Steps to reproduce

    @Test
    public void testEmpty() {
        clock.reset();
        final UnsortedClockFilter filter = new UnsortedClockFilter("Timestamp", clock, true);

        final Table result = testInput3.where("false").where(filter);

        final ControlledUpdateGraph updateGraph = ExecutionContext.getContext().getUpdateGraph().cast();
        updateGraph.runWithinUnitTestCycle(() -> {
            clock.run();
            filter.run();
        });

        assertEquals(0, result.size());
    }

Expected results

An empty table at the end of the run.

Actual results

java.lang.NullPointerException
	at io.deephaven.engine.table.impl.select.UnsortedClockFilter.updateAndGetAddedIndex(UnsortedClockFilter.java:127)
	at io.deephaven.engine.table.impl.select.ClockFilter.run(ClockFilter.java:125)
	at io.deephaven.engine.table.impl.select.TestClockFilters.lambda$testEmpty$12(TestClockFilters.java:209)
	at io.deephaven.engine.updategraph.impl.PeriodicUpdateGraph.runWithinUnitTestCycle(PeriodicUpdateGraph.java:636)
	at io.deephaven.engine.updategraph.impl.PeriodicUpdateGraph.runWithinUnitTestCycle(PeriodicUpdateGraph.java:617)
	at io.deephaven.engine.table.impl.select.TestClockFilters.testEmpty(TestClockFilters.java:207)

Additional details and attachments

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Versions

• Deephaven: 759c0f0