Preserve omitted assistant content during OpenAI-compatible message round-tripping

[i386]

Overview

Preserve omitted assistant content during OpenAI-compatible message round-tripping, instead of converting it to an empty string.

Incoming OpenAI-compatible message
{"role":"assistant","tool_calls":[...]}

Before
parse -> common_chat_msg(content = "")
serialize for chat template -> {"role":"assistant","content":"","tool_calls":[...]}

After
parse -> common_chat_msg(content_present = false)
serialize for chat template -> {"role":"assistant","tool_calls":[...]}

Requirements

• YES I have read and agree with the contributing guidelines
• AI usage disclosure: YES, I used AI to find this bug, fix the code and I have manually reviewed it. Was found when running a benchmark against custom llama improvements.

[ggml-gh-bot]

Hi @i386, thanks for your contribution!

Per our contribution guidelines, the automated PR checker found the following issue(s) that need your attention:

• Multiple open PRs from a new contributor: We limit new contributors (those without a previously merged PR) to 1 open PR at a time. You currently have 2 open PRs.

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