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ARROW-10639: [Rust] Added examples to is_null kernel and simplified signature.

The change in signature was motivated while writing the example: there is no reason to wrap a concrete array on an `Arc` just to be able to pass it to the kernel. All kernels require an immutable reference to anything that implements `Array`, and thus asking for `&Arc<dyn Array>` seems to be equivalent / worse as asking for `&Vec<>` instead of `&[]` that clippy complaints about.

Apart from that, this adds the examples.

Closes #8701 from jorgecarleitao/is_null_array

Authored-by: Jorge C. Leitao <jorgecarleitao@gmail.com>
Signed-off-by: Andrew Lamb <andrew@nerdnetworks.org>