Fix null content for chat-template. (#75)

The nullptr cause `Test failed: basic_string: construction from null is not valid`.
Change to use json j_null instead.

Tests pass with `./scripts/tests.sh`

Author: yichunk

include/minja/chat-template.hpp

@@ -165,11 +165,12 @@ class chat_template {
         auto out_empty = try_raw_render(json::array({dummy_user_msg, {{"role", "assistant"}, {"content", ""}}}), {}, false);
         auto out_null = try_raw_render(json::array({dummy_user_msg, {{"role", "assistant"}, {"content", nullptr}}}), {}, false);
         caps_.requires_non_null_content = contains(out_empty, user_needle) && !contains(out_null, user_needle);
-
+        
+        json j_null;
         auto make_tool_calls_msg = [&](const json & tool_calls) {
             return json {
                 {"role", "assistant"},
-                {"content", caps_.requires_non_null_content? "" : nullptr},
+                {"content", caps_.requires_non_null_content? "" : j_null},
                 {"tool_calls", tool_calls},
             };
         };
@@ -235,7 +236,7 @@ class chat_template {
                 };
                 const json tool_call_msg {
                     {"role", "assistant"},
-                    {"content", caps_.requires_non_null_content ? "" : nullptr},
+                    {"content", caps_.requires_non_null_content ? "" : j_null},
                     {"tool_calls", json::array({
                         {
                             // TODO: detect if requires numerical id or fixed length == 6 like Nemo