fix: jans-config-api /stat does not require month

[devrimyatar]

Describe the bug
When querying /jans-config-api/api/v1/stat parameter month is not mandatory.

[devrimyatar]

See sample query

https --verify=no -A bearer -a eyJraWQiOiIxNGF...D3HNAxB5COw "https://snap.gluu.org/jans-config-api/api/v1/stat?start_month=202208&end_month=202209"
HTTP/1.1 200 OK
Connection: Keep-Alive
Content-Length: 212
Content-Type: application/json
Date: Sat, 24 Sep 2022 11:45:08 GMT
Keep-Alive: timeout=5, max=100
Server: Apache/2.4.41 (Ubuntu)
Strict-Transport-Security: max-age=31536000; includeSubDomains
X-Content-Type-Options: nosniff
X-Xss-Protection: 1; mode=block

[
    {
        "month": "202209",
        "monthly_active_users": 1,
        "token_count_per_granttype": {
            "client_credentials": {
                "access_token": 9
            },
            "urn:ietf:params:oauth:grant-type:device_code": {
                "access_token": 1,
                "id_token": 1,
                "refresh_token": 1
            }
        }
    }
]