ARROW-10169: [Rust] Pretty print null PrimitiveTypes as empty strings
ARROW-10169: [Rust] Pretty print null PrimitiveTypes as empty strings

Null values should be printed as "" when pretty printing. Prior to this PR, , null values in primitive arrays  were rendered as the type's default value as pointed out by @jhorstmann  on https://github.com/apache/arrow/pull/8331#issuecomment-702964608

Closes #8332 from alamb/alamb/ARROW-10169-fix-null-in-pretty-print

Authored-by: alamb <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@nerdnetworks.org>
Signed-off-by: Jorge C. Leitao <dfffc62510208f06492f17a48dc6ce87aa15abf6@gmail.com>

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