Removing unused imports leaves extra blank line after package statement, isn't idempotent

I have a class that if I format it twice with google java formatter it will produce 2 different results. This is not what I would expect - I would think that you can format a file any number of times and google java formatter should always produce the exact same result. Is my assumption wrong ?

Here is an example:

original class:
```
$ cat inital_class.java
package com.sun.something;

import com.x;
import com.y;
import com.z;

public class MockedLiveServiceExecutionContext {

}
$
```

If I then format it I get this:
```
$ java -jar ./google-java-format-1.9-all-deps.jar --replace inital_class.java
$ cat inital_class.java
package com.sun.something;

public class MockedLiveServiceExecutionContext {}
$ cat inital_class.java | cksum
42955951 79
$
```

And if I format it again I get this (it's removed an extra space):
```
$ java -jar ./google-java-format-1.9-all-deps.jar --replace inital_class.java
$ cat inital_class.java
package com.sun.something;

public class MockedLiveServiceExecutionContext {}
$ cat inital_class.java | cksum
2486624596 78
$
```

使用 inital_class.java 和所示的 --replace 命令，透過 google-java-format-1.9 重現此問題，然後再執行一次，以確認輸出會變更。追蹤 formatter 對 package 和已移除 import 空白行的處理；當重複格式化相同檔案會產生完全相同的輸出時，即表示完成。

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