v3: leading newline gets removed

[natasha41575]

When writing a file such as

a:
  b:
    c: d

The newline at the top of the file is not preserved. It appears that this only happens with the first leading newline; if there are multiple leading newlines only one is removed.

This started happening after 496545a; prior to this commit the leading newline was preserved correctly.

[natasha41575]

#753 has a fix for this in its last commit.

I have also made a separate PR #756 to add a test and address this issue specifically.