fix: handle nil pointer exception in chat_prompt_template

There may be situation where promptValue is nil which will cause panic while accessing promptValue.Messages()

func (p ChatPromptTemplate) FormatMessages(values map[string]any) ([]llms.ChatMessage, error) {
    promptValue, err := p.FormatPrompt(values)
    if promptValue == nil {
        return nil, err
    }
    return promptValue.Messages(), err
}

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tmc / langchaingo

fix: handle nil pointer exception in chat_prompt_template #879

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