Add a Trigger → Issue event → Issue created

Next to the Issue event click on Add → Condition → Boolean condition

%{trigger.issue.assignee} = null

Next to the boolean condition click on Add → Action→ Assign issue

{
    "name": "Auto assign issues to your team",
    "description": "",
    "creator": "admin",
    "status": false,
    "triggerData": "1",
    "triggerType": "ISSUE_EVENT",
    "configuration": {
        "refs": [
            "issue",
            "system",
            "trigger.issue",
            "trigger.parent"
        ],
        "triggerType": "1"
    },
    "children": [
        {
            "sequence": 0,
            "type": "BOOLEAN_CONDITION",
            "ruleEntityType": "CONDITION",
            "configuration": {
                "refs": [
                    "issue",
                    "project",
                    "system",
                    "trigger",
                    "trigger.issue",
                    "trigger.parent"
                ],
                "expression": "%{trigger.issue.assignee} = null",
                "expressionParsingMode": "logical",
                "actingUser": "field_00020"
            },
            "children": [
                {
                    "sequence": 0,
                    "type": "ASSIGN_ISSUE",
                    "ruleEntityType": "ACTION",
                    "configuration": {
                        "refs": [
                            "issue",
                            "project",
                            "system",
                            "trigger",
                            "trigger.issue",
                            "trigger.parent"
                        ],
                        "assignMode": "projectRole",
                        "projectRole": "10001",
                        "mode": "projectRoleRandom",
                        "sendMail": "true",
                        "actingUser": "field_00020"
                    },
                    "children": null,
                    "hasChildren": false
                }
            ],
            "hasChildren": true
        }
    ],
    "hasChildren": true
}