serialize to char* with outputSize do not have terminator if the output is too small

[pat1]

Describe the bug
calling:
size_t serializeJson(const JsonDocument& doc, char* output, size_t outputSize);

if output is too small it do not have the terminator

Troubleshooter report
Here is the report generated by the ArduinoJson Troubleshooter:

1. The issue happens at run time
2. The issue concerns serialization
3. Output is incomplete
4. Program doesn't call serializeJson() in a loop
5. JsonDocument::overflowed() returns false
6. No string contains a NUL

Environment
Here is the environment that I used:

• Microcontroller: avr atmega1284p
• Core/runtime: platformio

Reproduction
to check the ploblem I have to do something like this:

    if (serializeJson(json,json_buffer, json_buffer_length) == json_buffer_length){
      json_buffer[0]='\0';
    }

[bblanchon]

Hi @pat1,

I'm not sure we can qualify that as a bug.
Sure, it's error-prone, just like strncpy(), but changing the behavior would break this legitimate use case:

size_t n = measureJson(doc);
char* json = malloc(n);
serializeJson(doc, json, n);
send(json, n);
free(n);

or, as I often see in Arduino code:

size_t n = measureJson(doc);
char json[n];
serializeJson(doc, json, n);
send(json, n);

However, I think this caveat should be documented, so I added a warning in the documentation.

Best regards,
Benoit