Refactor function name (#11210)

### What problem does this PR solve?

As title

### Type of change

- [x] Refactoring

---------

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

rag/utils/redis_conn.py

@@ -104,6 +104,7 @@ class RedisDB:
 
         if self.REDIS.get(a) == b:
             return True
+        return False
 
     def info(self):
         info = self.REDIS.info()
@@ -124,7 +125,7 @@ class RedisDB:
 
     def exist(self, k):
         if not self.REDIS:
-            return
+            return None
         try:
             return self.REDIS.exists(k)
         except Exception as e:
@@ -133,7 +134,7 @@ class RedisDB:
 
     def get(self, k):
         if not self.REDIS:
-            return
+            return None
         try:
             return self.REDIS.get(k)
         except Exception as e: