fix: error with gfa export of fully duplicated paths

[mmolari]

Fixes an error that would come up when exporting to gfa format with the --no-duplications flag, if one of the paths is composed of only duplicated blocks.
In this case the list of exported nodes is empty, and I would hit the error in line 151 of the old version, when nodes[1] was called.
I fixed it by simply avoid defining the path if no nodes are exported, and later avoid writing a path to file if undefined.

[nnoll]

Looks great. Correctly handles the empty case now.