ignore the first turn to apply "history" mechanism (#14118)

This will generate a meaningless string "system: " for generating
condense question; this increases the probability to make an improper
condense question and misunderstand user's question. Below is a case
- Original Question: Can you explain the arguments of Meilisearch?
- Condense Question
  - What are the benefits of using Meilisearch? (by CodeLlama)
  - What are the reasons for using Meilisearch? (by GPT-4)

The condense questions (not matter from CodeLlam or GPT-4) are different
from the original one.

By checking the content of each dialogue turn, generating history string
only when the dialog content is not empty.
Since there is nothing before first turn, the "history" mechanism will
be ignored at the very first turn.

Doing so, the condense question will be "What are the arguments for
using Meilisearch?".

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Co-authored-by: Bagatur <baskaryan@gmail.com>
Co-authored-by: Bagatur <22008038+baskaryan@users.noreply.github.com>
Co-authored-by: ccurme <chester.curme@gmail.com>

libs/langchain/langchain/chains/conversational_retrieval/base.py

@@ -42,8 +42,11 @@ def _get_chat_history(chat_history: List[CHAT_TURN_TYPE]) -> str:
     buffer = ""
     for dialogue_turn in chat_history:
         if isinstance(dialogue_turn, BaseMessage):
-            role_prefix = _ROLE_MAP.get(dialogue_turn.type, f"{dialogue_turn.type}: ")
+            if len(dialogue_turn.content) > 0:
-            buffer += f"\n{role_prefix}{dialogue_turn.content}"
+                role_prefix = _ROLE_MAP.get(
+                    dialogue_turn.type, f"{dialogue_turn.type}: "
+                )
+                buffer += f"\n{role_prefix}{dialogue_turn.content}"
         elif isinstance(dialogue_turn, tuple):
             human = "Human: " + dialogue_turn[0]
             ai = "Assistant: " + dialogue_turn[1]