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ARROW-7008: [C++] Check binary offsets and data buffers for nullness in validation. Produce valid arrays in DictionaryEncode on zero-length arrays

A couple of related issues:

* Validate() and ValidateFull() did not check the offsets and data buffers of BinaryArray for nullness. We might relax the requirements of the columnar format docs to allow them to be null, but that should be a separate matter
* DictionaryEncode (and other hash kernels) were producing invalid dictionaries for zero-length BinaryArray input

Closes #6803 from wesm/ARROW-7008

Authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Wes McKinney <wesm+git@apache.org>