Fix panic when multiply assign

[care0717]

I fix #1256

When multi documents exist, I've got panic because of expressionNode.Operation.Preferences = nil.
But I've got type assertion error instead of this issue when I remove it. So I make type assertion safe.

[care0717]

I'm wondering what kind of test cases to add or not.

Before I fix it, this test case is passed.

	{
		skipDoc:    true,
		document:   "a: 1\n---\nb: 2",
		expression: `. *= {"c": 3}`,
		expected: []string{
			"D0, P[], (!!map)::a: 1\nc: 3\n",
			"D1, P[], (!!map)::b: 2\nc: 3\n",
		},
	},

Unlike stream_evaluator, operator_test push all nodes to inputList before GetMatchingNodes. (maybe I think)

Let me know what you think.

[mikefarah]

Wow this is a really good find! You are right about stream_evaluator vs operator_test as well. I think the best way to test this case (for now) is to add an acceptance test. I'm happy to do that.