community[patch]: change default Neo4j username/password (#25226)

**Description:**

Change the default Neo4j username/password (when not supplied as
environment variable or in code) from `None` to `""`.

Neo4j has an option to [disable
auth](https://neo4j.com/docs/operations-manual/current/configuration/configuration-settings/#config_dbms.security.auth_enabled)
which is helpful when developing. When auth is disabled, the username /
password through the `neo4j` module should be `""` (ie an empty string).

Empty strings get marked as false in
`langchain_core.utils.env.get_from_dict_or_env` -- changing this code /
behaviour would have a wide impact and is undesirable.

In order to both _allow_ access to Neo4j with auth disabled and _not_
impact `langchain_core` this patch is presented. The downside would be
that if a user forgets to set NEO4J_USERNAME or NEO4J_PASSWORD they
would see an invalid credentials error rather than missing credentials
error. This could be mitigated but would result in a less elegant patch!

**Issue:**
Fix issue where langchain cannot communicate with Neo4j if Neo4j auth is
disabled.

libs/community/langchain_community/graphs/neo4j_graph.py

@@ -346,18 +346,27 @@ class Neo4jGraph(GraphStore):
             )
 
         url = get_from_dict_or_env({"url": url}, "url", "NEO4J_URI")
-        username = get_from_dict_or_env(
-            {"username": username}, "username", "NEO4J_USERNAME"
-        )
-        password = get_from_dict_or_env(
-            {"password": password}, "password", "NEO4J_PASSWORD"
-        )
+        # if username and password are "", assume Neo4j auth is disabled
+        if username == "" and password == "":
+            auth = None
+        else:
+            username = get_from_dict_or_env(
+                {"username": username},
+                "username",
+                "NEO4J_USERNAME",
+            )
+            password = get_from_dict_or_env(
+                {"password": password},
+                "password",
+                "NEO4J_PASSWORD",
+            )
+            auth = (username, password)
         database = get_from_dict_or_env(
             {"database": database}, "database", "NEO4J_DATABASE", "neo4j"
         )
 
         self._driver = neo4j.GraphDatabase.driver(
-            url, auth=(username, password), **(driver_config or {})
+            url, auth=auth, **(driver_config or {})
         )
         self._database = database
         self.timeout = timeout