Helper method for Item_func_json_* methods. Check if a JSON item or JSON text is valid and, for the latter, optionally construct a DOM tree (i.e. only if valid). @param[in] args Item_func::args alias @param[in] arg_idx Index (0-based) of argument into the args array @param[out] value Item_func_json_*::m_value alias @param[in] func_name Name of the user-invoked
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