EMPTY_BLOCK_STRUCTURE_ERROR shouldn't trigger on empty lambdas

[petertrr]

Describe the bug

Sometimes empty lambda can be used as a function that does nothing:

fun foo(bar: () -> Unit = {})

or empty function in implementation of functional interface:

A({})  // instead of A(Disposable {}), where Disposable is functional interface from Java

Diktat shouldn't issue a warning in these cases, because they are valid and can't be done differentlty. Also, kotlin copiler warns that Unit is redundant in { Unit }.

Environment information

• diktat version: 0.4.2