Handle significant whitespace before a blocks body

[Blacksmoke16]

Follow up to #15568 to handle the case of significant whitespace before a block's body.

Previously code like:

{%
  ({"a" => "b"} of Nil => Nil).each do |k, v|
    # stuff and things
    k + v

    # foo bar

    k + v
  end
%}

Would always be stringified as:

{%
  ({"a" => "b"} of Nil => Nil).each do |k, v|
    k + v



    k + v
  end
%}

But now results in:

{%
  ({"a" => "b"} of Nil => Nil).each do |k, v|
  
    k + v
  
  
  
    k + v
  end
%}