tools/trace: Don't crash if a trace has no timing information.

src/gallium/tools/trace/model.py

@@ -230,8 +230,9 @@ class PrettyPrinter:
         if node.ret is not None:
             self.formatter.text(' = ')
             node.ret.visit(self)
-        self.formatter.text(' // time ')
-        node.time.visit(self)
+        if node.time is not None:
+            self.formatter.text(' // time ')
+            node.time.visit(self)
 
     def visit_trace(self, node):
         for call in node.calls:

src/gallium/tools/trace/parse.py

@@ -214,7 +214,7 @@ class TraceParser(XmlParser):
         method = attrs['method']
         args = []
         ret = None
-        time = 0
+        time = None
         while self.token.type == ELEMENT_START:
             if self.token.name_or_data == 'arg':
                 arg = self.parse_arg()