Passing zero arguments as params -- where the behaviour is defined?

17.5.1.4 Parameter arrays

A parameter array permits arguments to be specified in one of two ways in a method invocation:

• The argument given for a parameter array can be a single expression of a type that is implicitly convertible (§13.1) to the parameter array type. In this case, the parameter array acts precisely like a value parameter.

• Alternatively, the invocation can specify zero or more arguments for the parameter array, where each argument is an expression of a type that is implicitly convertible (§13.1) to the element type of the parameter array. In this case, the invocation creates an instance of the parameter array type with a length corresponding to the number of arguments, initializes the elements of the array instance with the given argument values, and uses the newly created array instance as the actual argument.

In the same section an example is given:

using System;
class Test
{
    static void F(params int[] args) {
        Console.Write("Array contains {0} elements:", args.Length);
        foreach (int i in args)
            Console.Write(" {0}", i);
        Console.WriteLine();
    }

    static void Main() {
        int[] arr = {1, 2, 3};
        F(arr);
        F(10, 20, 30, 40);
        F();
    }
}

produces the output

Array contains 3 elements: 1 2 3 Array
contains 4 elements: 10 20 30 40 Array
contains 0 elements:

This example illustrates the expected behavior: empty array

Section 7.4.1 of the C# Language Specification (ref: C# 3.0 spec)

In particular, note that an empty array is created when there are zero arguments given for the parameter array.

It's the last line of the section