[Bug][Outfile] Fix bug that column separater is missing in output file. (#3765)

When output result of a query using `OUTFILE` statement, is some of output
column is null, then then following column separator is missing.

be/src/runtime/file_result_writer.cpp

@@ -149,6 +149,9 @@ Status FileResultWriter::_write_one_row_as_csv(TupleRow* row) {
 
             if (item == nullptr) {
                 _plain_text_outstream << NULL_IN_CSV;
+                if (i < num_columns - 1) {
+                    _plain_text_outstream << _file_opts->column_separator;
+                }
                 continue;
             }
 

docs/en/administrator-guide/outfile.md

@@ -184,3 +184,5 @@ mysql> SELECT * FROM tbl INTO OUTFILE ...
 * The timeout of the export command is the same as the timeout of the query. It can be set by `SET query_timeout = xxx`.
 * For empty result query, there will be an empty file.
 * File spliting will ensure that a row of data is stored in a single file. Therefore, the size of the file is not strictly equal to `max_file_size`.
+* For functions whose output is invisible characters, such as BITMAP and HLL types, the output is `\N`, which is NULL.
+* At present, the output type of some geo functions, such as `ST_Point` is VARCHAR, but the actual output value is an encoded binary character. Currently these functions will output garbled characters. For geo functions, use `ST_AsText` for output.

docs/zh-CN/administrator-guide/outfile.md

@@ -180,3 +180,5 @@ mysql> SELECT * FROM tbl INTO OUTFILE ...
 * 导出命令的超时时间同查询的超时时间。可以通过 `SET query_timeout=xxx` 进行设置。
 * 对于结果集为空的查询，依然会产生一个大小为0的文件。
 * 文件切分会保证一行数据完整的存储在单一文件中。因此文件的大小并不严格等于 `max_file_size`。
+* 对于部分输出为非可见字符的函数，如 BITMAP、HLL 类型，输出为 `\N`，即 NULL。
+* 目前部分地理信息函数，如 `ST_Point` 的输出类型为 VARCHAR，但实际输出值为经过编码的二进制字符。当前这些函数会输出乱码。对于地理函数，请使用 `ST_AsText` 进行输出。