fix for segfault when null hunks are passed to callbacks

[xloem]

I'm not certain what the "right" solution for this is, but as-is there is a segfault when diffs are printed.

A null hunk pointer is passed from https://github.com/libgit2/libgit2/blob/main/src/libgit2/diff_print.c#L164 via https://github.com/bold84/cppgit2/blob/master/src/diff.cpp#L199 to the hunk constructor, causing dereference of null.

This quick patch changes the hunk constructor to initialise to zeros if passed a null value.

This breaks parallelism with line and delta, which do not have such a check, but it prevents the segfault.