use empty strand when seqnames are also empty, closes #81

[hpages]

Thanks Marcel.

The fix should just be to replace this line

ans_strand <- "*"

with this line

ans_strand <- NULL

here (note that NULL is the default for the strand argument of the GRanges() constructor).

Also can you please remove the new example from the man page? It's not that educative or interesting.

Please post a short comment in the issue or assign it to you if you decide to tackle an issue like this so we don't duplicate efforts. Thanks!