installer: let answerfile accept None as to mean "add nothing"

This is useful for a lambda passed to @pytest.mark.answerfile, where in
some variants of a test we want to add an element, but nothing in other
variants (eg. a <raid> block)

Signed-off-by: Yann Dirson <[email protected]>

Author: ydirson

lib/installer.py

@@ -19,6 +19,8 @@ def write_xml(self, filename):
 
     def top_append(self, *defs):
         for defn in defs:
+            if defn is None:
+                continue
             self.defn['CONTENTS'].append(self._normalize_structure(defn))
         return self