Fix struct binary search and struct flattening

[ttnghia]

This PR fixes several bugs. In particular:

• Fixes a bug in struct binary search that only check for null elements at the top level.
• Fixes a bug in struct flattening that uses default null order for the children of the input structs columns.

Unit tests for struct binary search and struct sorting are also rewritten.
Closes #8189.

[codecov]

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[revans2]

From my perspective things look good. I have rerun tests and they pass.

[harrism]

@gpucibot merge