[css-mixins] FunctionParameter dictionary should not have any required members

[weinig]

The FunctionParameter dictionary type is only used as a return value so should follow the rule from WebIDL that states:

"Specification authors should leave the members optional in all other cases, including when a dictionary type is used solely as the return type of operations." (https://webidl.spec.whatwg.org/#dfn-dictionary).

(Alternatively, if it makes more sense to ignore that "should" rule, all the members should be required, as the prose dictates that each has specific values, none of which are the value "undefined").

[weinig]

An additional point here is that the WPT test, https://wpt.fyi/results/css/css-mixins/at-function-cssom.html, expects the dictionaries returned from CSSFunctionRule getParameters() to not have a defaultValue field at all if no default was specified, implying that the spec text that reads:

"The default value of the function parameter, or null if the argument does not have a default"

is not correct, and returning null is wrong.