Fix the issue where ANTHROPIC_API_URL set in environment is not takin… (#6400)

I apologize for the error: the 'ANTHROPIC_API_URL' environment variable
doesn't take effect if the 'anthropic_api_url' parameter has a default
value.

#### Who can review?
  Models
  - @hwchase17
  - @agola11

langchain/llms/anthropic.py

@@ -36,7 +36,7 @@ class _AnthropicCommon(BaseModel):
     default_request_timeout: Optional[Union[float, Tuple[float, float]]] = None
     """Timeout for requests to Anthropic Completion API. Default is 600 seconds."""
 
-    anthropic_api_url: str = "https://api.anthropic.com"
+    anthropic_api_url: Optional[str] = None
 
     anthropic_api_key: Optional[str] = None
 
@@ -52,7 +52,10 @@ class _AnthropicCommon(BaseModel):
         )
         """Get custom api url from environment."""
         anthropic_api_url = get_from_dict_or_env(
-            values, "anthropic_api_url", "ANTHROPIC_API_URL"
+            values,
+            "anthropic_api_url",
+            "ANTHROPIC_API_URL",
+            default="https://api.anthropic.com",
         )
 
         try: