Error occurs when executing `FindDocInfosByKeys` query with empty docKeys argument

[raararaara]

What happened: When performing the FindDocInfosByKeys query with an empty array for docKeys, the server encounters a critical issue. Specifically, the server crashes with the following error:

2024-07-26T01:27:13.710+0900    WARN    a16     RPC : "/yorkie.v1.AdminService/GetDocuments" 
1.773583ms: &{"project_name:\"default\"" {'\x00' "/yorkie.v1.AdminService/GetDocuments" 
%!q(bool=false) '\x00'} {"[::1]:49484" "connect" map[]} map["Authorization":
["cqh6cnh97i624rnkavk0"] "Content-Length":["70"] "Content-Type":["application/json"] "Host":
["localhost:8080"]] "POST"} => "find documents: (BadValue) $in needs an array"

How to reproduce it (as minimally and precisely as possible):

1. Execute the FindDocInfosByKeys query with an empty array for docKeys.
2. Observe the server crashing and generating the mentioned error.

Anything else we need to know?:

Environment:

• Operating system:
• Browser and version:
• Yorkie version (use yorkie version): v0.4.28
• Yorkie JS SDK version:

[raararaara]

cc @blurfx