[Bugfix][Frontend] Strip empty tool calls from incoming chat conversations

[benchislett]

The Llama chat templates cannot handle an assistant message with the tool_calls field set but equal to an empty list. Most other chat templates just loop over the tool_calls if there are any, but the Llama chat template asserts that if the field is present, the list's length is exactly 1. This causes an error when the list is given, but is empty.

To resolve this issue, I just ignore the field when an "assistant" message has len(tool_calls) == 0. Also added a regression test to reproduce the issue and confirm the patch is successful.

FIX #13978

[github-actions]

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[DarkLight1337]

cc @K-Mistele

[aarnphm]

LGTM