improve the readability of the function generate_api_key (#12552)

api/models/model.py

     def generate_api_key(prefix, n):
         while True:
             result = prefix + generate_string(n)
-            while db.session.query(ApiToken).filter(ApiToken.token == result).count() > 0:
-                result = prefix + generate_string(n)
-
+            if db.session.query(ApiToken).filter(ApiToken.token == result).count() > 0:
+                continue
             return result