`DOMMatrix.multiply` should allow undefined argument

[Finesse]

The specification says:

If otherMatrix is omitted, the matrix is multiplied by a matrix in which every element is 0 except the bottom-right corner and the element immediately above and to its left: m33 and m34. These have the default value of 1.

In fact, the polyfill throws an error: «The argument to multiplySelf must be an instance of DOMMatrix».

Using new DOMMatrix() as the default argument value should solve this issue.