Fix Visit_Aggregation on ASTTemplate empty count handling

[mla2001]

Initial Checks

• I have searched existing issues for duplicates
• I have read the documentation

Summary

Currently, the Visit_Aggregation method on ASTTemplate fails with empty count handling, so inheritors would fail too.

Reproducible Example

script = "DS_r <- DS_1[aggr Me_1 := count()];"

    data_structures = {
        "datasets": [
            {
                "name": "DS_1",
                "DataStructure": [
                    {
                        "name": "Id_1",
                        "type": "Integer",
                        "role": "Identifier",
                        "nullable": False,
                    },
                ],
            }
        ]
    }

    ast = create_ast(script)
    ASTTemplate().visit(ast)

vtlengine version

1.6.1

Python version

Any

OS

Any