Assertion in my_strnxfrm_unicode_full_bin() fails once JSON string in
JSON_UNQUOTE is empty. Item_func_json_unquote::val_str() appends the empty
string to m_value with Json_wrapper.to_string(). But Json_wrapper.to_string()
does nothing to m_value because of the empty string. If m_value is not
initialized before, its data pointer is NULL.

Fix:
Return make_empty_result() if m_value.length() is zero.

Change-Id: I18e59b716d8b480cd0c66754a427552dce68fd3a