[fix](multi table) do not use strlen to calculate the length of msg (#40367) (#40511)

pick #40367

Meet code dump when using single stream multi table load:
```
SUMMARY: AddressSanitizer: heap-buffer-overflow /root/doris/be/src/io/fs/multi_table_pipe.cpp:99:22 in doris::io::MultiTablePipe::dispatch(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>> const&, char const*, unsigned long, doris::Status (doris::io::KafkaConsumerPipe::*)(char const*, unsigned long))
```

1. It is hard to guaranteed that msg is a C-style string ending in '\0'
character. If not, it may cause the core dump to access memory out of
bounds.
2. It is not need to calculate the length of msg twice.

Therefore, deleting the logic that using strlen to calculate the length
of msg.

be/src/io/fs/multi_table_pipe.cpp

@@ -96,7 +96,7 @@ std::string MultiTablePipe::parse_dst_table(const char* data, size_t size) {
 
 Status MultiTablePipe::dispatch(const std::string& table, const char* data, size_t size,
                                 AppendFunc cb) {
-    if (size == 0 || strlen(data) == 0) {
+    if (size == 0) {
         LOG(WARNING) << "empty data for table: " << table << ", ctx: " << _ctx->brief();
         return Status::InternalError("empty data");
     }