Non-existent functions can't be mocked since 2.1.0

The problem was introduced here:
https://github.com/Codeception/AspectMock/pull/133/files#diff-50b2697c4aa86740dcae323b3d7ebe44R44

The `placeOptionalAndReferenceFunction` method uses `ReflectionFunction` which throws an exception if a non-existent function name is passed:

> [ReflectionException] Function somefunc() does not exist

Before that change it worked fine when trying to mock a non-existent function. This is a valid use case for a plugin architecture where you may not have desired functions in a test environment.

Can it be like this, for example:
```php
if (function_exists($function)) {
$this->placeOptionalAndReferenceFunction($namespace, $function);
}
```

从 placeOptionalAndReferenceFunction 方法和所引用的 pull request diff 开始，重点关注其对 ReflectionFunction 的调用。确认对于不存在的函数的行为，然后验证对这样的函数进行 mocking 不再引发 ReflectionException。

由索引模型根据 Issue 内容生成。