Low: libcrmcommon: always respect overwrite in pcmk__cmp_nvpair_blocks()

When pcmk__cmp_nvpair_blocks() compares scores, it returns a different
sorting value depending on whether unpack_data->overwrite is true.

However, it previously did not check overwrite when comparing NULL
values or when comparing the magic ID that should always sort first.
Now, it does.

This issue didn't matter internally because internal callers always
pass overwrite as FALSE, and the previous values were correct for that.