node_invoke() returns inconsistent types

Whilst doing some weird stuff at chx's suggestion, I stumbled across a problem in the node_invoke() function. node_invoke() first checks to see if a node module implements a given hook, then if it does it invokes it and returns the result. If it doesn't, however, then it returns nothing. Since node_invoke() is sometimes called from array_merge() or array_merge_recursive(), that causes a problem since the second parameter to array_merge() is then a NULL value rather than an array.

This patch fixes the problem by adding a default return array() to the function, so that it always returns an array as expected, even if the hook is not found.