fix: use "" instead of nullptr for templates requiring non null content

[grf53]

Description

Due to uses of nullptr to test templates which are caps_.requires_non_null_content == true, the capabilities of that template are being mis-estimated.
In the case I saw, even though the template covers 'tool call' and 'tool response', caps_.supports_tool_calls and caps_.supports_tool_responses were both false, so the tool calls and tool responses are rendered with the polyfills.

Currently in main, the non-null content requirement is being checked(caps_.requires_non_null_content), but it is difficult to find any meaningful use cases.
It seems desirable that this be used to suppress the above situation.

[grf53]

With the Qwen3 model's chat template being improved, it seems that the change in this PR is no longer mandatory for Qwen3 template.

However, it may still be meaningful for other templates that require non-null content.

[yichunk]

Could we merge this PR?
Though the qwen3 template is updated, this update is still useful.

[ochafik]

Thanks @grf53 for sending this out, and sorry for the delay!!

[yichunk]

This PR broke the chat-template tests?
./scripts/tests.sh results in all test cases failed with the error like

460/547 Test #471: test-supported-template-Qwen-Qwen2.5-1.5B-Instruct-tool_use .....................................................***Failed    0.24 sec
# Testing template:
# ./build/bin/test-supported-template ["minja/build/tests/Qwen-Qwen2.5-1.5B-Instruct.jinja","minja/build/tests/Qwen-Qwen2.5-1.5B-Instruct.caps.json","minja/tests/contexts/tool_use.json","minja/build/tests/Qwen-Qwen2.5-1.5B-Instruct-tool_use.txt"]
Test failed: basic_string: construction from null is not valid

[yichunk]

Please check the fix #75

[yatarkan]

This PR broke the chat-template tests?

That is true, tests were broken by this PR, see https://github.com/google/minja/actions/runs/15346704885/job/43184463260#step:9:7818