ARROW-12425: [Rust] Fix new_null_array dictionary creation
ARROW-12425: [Rust] Fix new_null_array dictionary creation

It is my understanding that an arrow array should always have a backing values array, even if the content is all nulls. new_null_array currently violates this as it doesn't allocate the backing store for DictionaryArrays. This causes the concat kernel, and possibly others, to panic with index violations

Signed-off-by: Raphael Taylor-Davies <d0bb0a6fd3446defe7e9f6ff7e79bb26786a04d9@googlemail.com>

Closes #10072 from tustvold/null-dictionary-creation

Authored-by: Raphael Taylor-Davies <d0bb0a6fd3446defe7e9f6ff7e79bb26786a04d9@googlemail.com>
Signed-off-by: Andrew Lamb <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@nerdnetworks.org>

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