Do not apply indent on empty lines.

This manifested when `BlockWriter` had no `Snippet`s to contribute but appended a trailing newline. The resulting code had trailing indent whitespace on after the open curly brace.
```java
public Foo() { // <-- trailing whitespace was here
}
```
Too opinionated? This forces interface methods to be explicitly abstract by affording empty methods from appearing abstract (#111).

I think it's good for now, but we should consider what #110 would mean in solving this.