ARROW-11102: [Rust][DataFusion] fmt::Debug for ScalarValue(Utf8) is always quoted
ARROW-11102: [Rust][DataFusion] fmt::Debug for ScalarValue(Utf8) is always quoted

This PR changes the `fmt::Debug` implementation for `ScalarValue(Utf8)` and `ScalarValue(LargeUtf8)` so that it is possible to differentiate `Utf8("NULL")` and `Utf8(NULL)` when viewing execution plans (as the string "NULL" may be a valid value).

Closes #9074 from seddonm1/fix-debug-utf8-null

Authored-by: Mike Seddon <08143e5b7f3ef9b2c05b03e8981fb36a6affd6c2@gmail.com>
Signed-off-by: Jorge C. Leitao <dfffc62510208f06492f17a48dc6ce87aa15abf6@gmail.com>

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