fix: correct function type for spyon an optional function

[li-yechao]

What does this PR do?

Mock type is incorrect when spyOn a nullable function, like this:

  const test: {
    optionalMethod?: (input: { question: string }) => { answer: string };
  } = {
    optionalMethod: (input) => ({ answer: `Aswer to ${input.question}` }),
  };

  const a = spyOn(test, "optionalMethod");

typeof a is Mock<never>

How did you verify your code works?