Addresses review comments.

Signed-off-by: Syed Tousif Ahmed <stahmed@seas.upenn.edu>

prjxray/node_model.py

diff --git a/prjxray/node_model.py b/prjxray/node_model.py
index 8a1b5ca..d187c57 100644
--- a/prjxray/node_model.py
+++ b/prjxray/node_model.py
@@ -45,7 +45,7 @@
 
         self.nodes = None
 
-        self.wire_to_node_map = {}
+        self.wire_to_node_map = None
 
         if progressbar is None:
             self.progressbar = lambda x: x
@@ -143,6 +143,8 @@
         return self.nodes[tile, wire]
 
     def _build_wire_to_node_map(self):
+        self.wire_to_node_map = {}
+
         if self.nodes is None:
             self._build_nodes()
 
@@ -153,7 +155,7 @@
 
     def get_node_for_wire(self, tile, wire):
         """ Get node for specified tile and wire. """
-        if not self.wire_to_node_map:
+        if self.wire_to_node_map is None:
             self._build_wire_to_node_map()
 
         return self.wire_to_node_map[tile, wire]