Fix 'is' operator for comparison

The 'is' operator should not be used for comparison on some types of
literals. In CPython this works by accident and CPython 3.8 introduced a
SyntaxWarning (see https://bugs.python.org/issue34850).

Change-Id: Ifabe22e4ae3b9e1d3bfd5a663f39500e381cfa6e
Reviewed-on: https://gerrit.libreoffice.org/83034
Tested-by: Jenkins
Reviewed-by: Xisco Faulí <xiscofauli@libreoffice.org>

wizards/com/sun/star/wizards/common/FileAccess.py

@@ -129,7 +129,7 @@ class FileAccess(object):
 
     def getPath(self, parentURL, childURL):
         string = ""
-        if childURL is not None and childURL is not "":
+        if childURL is not None and childURL != "":
             string = "/" + childURL
         return self.filenameConverter.getSystemPathFromFileURL(
             parentURL + string)