[fix](csv reader) fix csv parser incorrect if enclosing line_delimiter (

#38347)

Csv reader parse data incorrect when data enclosing line_delimiter, for
example, line_delimiter is \n and enclose is ', data as follows:
```
'aaaaaaaaaaaa
bbbb'
```
it will be parsed as two columns: `'aaaaaaaaaaaa` and `bbbb',` rather
than one column
```
'aaaaaaaaaaaa
bbbb'
```

The reason why this happened is csv reader will not reset result when
not match enclose in this `output_buf_read`, causing incorrect
truncation was made.

Co-authored-by: Xin Liao <[email protected]>

be/src/vec/exec/format/file_reader/new_plain_text_line_reader.cpp

@@ -160,6 +160,11 @@ void EncloseCsvLineReaderContext::_on_pre_match_enclose(const uint8_t* start, si
         if (_idx != _total_len) {
             len = update_reading_bound(start);
         } else {
+            // It needs to set the result to nullptr for matching enclose may not be read
+            // after reading the output buf.
+            // Therefore, if the result is not set to nullptr,
+            // the parser will consider reading a line as there is a line delimiter.
+            _result = nullptr;
             break;
         }
     } while (true);