Clean up syntax to be more idiomatic

The following cleans up some non-idiomatic code with in python.

Author: SimonRichardson

juju/origin.py

@@ -61,9 +61,7 @@ def __init__(self, track=None, risk=None):
         if not Risk.valid(risk):
             raise JujuError("unexpected risk {}".format(risk))
 
-        if track is None:
-            track = ""
-        self.track = track
+        self.track = track or ""
         self.risk = risk
 
     @staticmethod
@@ -77,8 +75,6 @@ def parse(s):
 
         p = s.split("/")
 
-        print(len(p))
-
         risk = None
         track = None
         if len(p) == 1:
@@ -163,7 +159,7 @@ def parse(s):
         else:
             raise JujuError("platform is malformed and has too many components {}".format(s))
 
-        if arch is None or arch == "":
+        if not arch:
             raise JujuError("architecture in platform {} is not valid".format(s))
         if os is not None and os == "":
             raise JujuError("os in platform {} is not valid".format(s))