fix: handle None content in get_cur_message_text

[dajiaohuang]

Fixes #5649

Problem

When an assistant message has no text content (e.g., image-only responses or rejections), msg[content] is None. This causes a TypeError in get_cur_message_text() when attempting string concatenation with None.

Solution

Added a null check before concatenating message content:

if msg["content"] is not None:
    text += msg["content"] + "\n"

Test

Added test cases in tests/basic/test_coder.py to verify handling of messages with None content.

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