alternator: cleanup of get_string_attribute() function
alternator: cleanup of get_string_attribute() function

The get_string_attribute() function used attribute_value->GetString()
to return an std::string. But this function does not actually return a
std::string - it returns a char*, which gets implicitly converted to
an std::string by looking for the first null character. This lookup is
unnecessary, because rjson already knows the length of the string, and
we can use it.

This patch is just a cleanup and a very small performance improvement -
I do not expect it fixes any bugs or changes anything functional, because
JSON strings anyway cannot contain verbatim nulls.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>
Message-Id: <20200219101159.26717-1-625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

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