Handle empty requirements.txt

Fixes bazelbuild/rules_python#36

rules_python/piptool.py

diff --git a/rules_python/piptool.py b/rules_python/piptool.py
index 4d8647e..73a0263 100644
--- a/rules_python/piptool.py
+++ b/rules_python/piptool.py
@@ -141,7 +141,7 @@
   name = name.replace("-", "_").lower()
   return _requirements[name]
 """.format(input=args.input,
-           whl_libraries='\n'.join(map(whl_library, whls)),
+           whl_libraries='\n'.join(map(whl_library, whls)) if whls else "pass",
            mappings=','.join([
              '"%s": "@%s//:pkg"' % (wheel.distribution().lower(), wheel.repository_name())
              for wheel in whls

tools/piptool.par

diff --git a/tools/piptool.par b/tools/piptool.par
index ba7fb26..f1087be 100755
--- a/tools/piptool.par
+++ b/tools/piptool.par
Binary files differ