tools: fix deprecated syntax.

This fixes a similar deprecation warning on various pieces of code:

> DeprecationWarning: Testing an element's truth value will always return True in future versions.  Use specific 'len(elem)' or 'elem is not None' test instead.
>   samples = log.find ("samples") or empty_element

tools/performance-log-coalesce.py

@@ -32,7 +32,7 @@ log = ElementTree.fromstring (sys.stdin.buffer.read ())
 
 address_map = log.find ("address-map")
 
-if address_map:
+if address_map is not None:
     addresses = []
 
     # Create base addresses dictionary