Fix null content for chat-template.

[yichunk]

The nullptr cause Test failed: basic_string: construction from null is not valid.

The root cause is the ternary operator of std::string and nullptr

Before C++23:
Constructing a std::string from a null pointer (like nullptr or NULL) results in Undefined Behavior. This means the program's behavior is unpredictable and can lead to crashes or incorrect results.
In C++23 and later:
Constructing a std::string from a null pointer is explicitly disabled and will result in a compilation error. This prevents the undefined behavior from occurring.

https://softwareengineering.stackexchange.com/questions/450335/is-it-bad-practice-to-use-nullptr-in-ternary-operation

Change to use empty json j_null instead.
https://json.nlohmann.me/api/basic_json/is_null/

Tests pass with ./scripts/tests.sh

[yichunk]

@ochafik for review