Fix gh-1293 for sum over zero-size array with keepdims set

[oleksandr-pavlyk]

Closes gh-1293

This PR adds handling of keepdims=True case for special-case early exit for zero-size input.

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpctl/pulls/1294/index.html

[coveralls]

coverage: 83.218% (+0.02%) from 83.203% when pulling a4e357f on fix-gh-1293-sum-keepdims-true into af302a5 on master.

[github-actions]

Array API standard conformance tests for dpctl=0.14.6dev0=py310h7bf5fec_12 ran successfully.
Passed: 475
Failed: 525
Skipped: 119

[antonwolfy]

I've tested with dpnp from #1488 and all tests are passing now. Thank you @oleksandr-pavlyk !

[github-actions]

Deleted rendered PR docs from intelpython.github.com/dpctl, latest should be updated shortly. 🤞

[github-actions]

Array API standard conformance tests for dpctl=0.14.6dev0=py310h7bf5fec_12 ran successfully.
Passed: 475
Failed: 525
Skipped: 119