Sanitize percentile_approx() output for empty input

[SrikarVanavasam]

Closes #10856

If all of the tdigest inputs to percentile_approx are empty, it will return a column containing null rows, as expected, but they will have unsanitary offsets. This PR checks if all the inputs are empty and returns an empty column as expected.

[codecov[bot]]

Codecov Report

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[nvdbaranec]

@gpucibot merge