Wrong comparison with empty string in version 0.18

[lscrd]

In version 0.18, the result of the comparison "a" > "" is false instead of true. The result is correct with version 0.17.2.

Even more strange - BOTH "a" > "" and "a" < "" output false!

[skilchen]

This bug was introduced by the following commit:

commit 0b52466bb83a747ee7c3325f20b741ad243c0c6a
Author: Andreas Rumpf <rumpf_a@web.de>
Date:   Fri Jan 19 09:26:11 2018 +0100

  bugfix: do not call memcmp for nil strings

I think, line 33 in lib/system/sysstr.nim should be changed from:

result = 0

to

result = a.len - b.len