fts: fts_open: do not let an empty string cause immediate failure

fts: fts_open: do not let an empty string cause immediate failure

This is required in support of GNU rm, for which the command
"rm A '' B" must process and remove both A and B, in spite of
the empty string argument.
* lib/fts.c (fts_open): Do not let the presence of an empty string
cause fts_open to fail immediately.  Most fts-using tools must be
able to process all arguments, in order, and can be expected to
diagnose such arguments themselves.
Also, move declaration of local, "len", "down" to initialization.