size in files is made optional (with commit 3922)

so we must handle it carefully while creating nodes not to get errors like below:

System Error. Program Terminated.
exceptions.TypeError: node contents must be a string

Traceback:
  File "/usr/bin/pisi", line 65, in ?
    cli.run_command()
  File "/usr/lib/python2.3/site-packages/pisi/cli/pisicli.py", line 107, in run_command
    self.command.run()
  File "/usr/lib/python2.3/site-packages/pisi/cli/commands.py", line 1029, in run
    pisi.api.build_until(arg, "buildpackages", self.authInfo)
  File "/usr/lib/python2.3/site-packages/pisi/api.py", line 327, in build_until
    __buildState_buildpackages(pb, last)
  File "/usr/lib/python2.3/site-packages/pisi/api.py", line 300, in __buildState_buildpackages
    pb.build_packages()
  File "/usr/lib/python2.3/site-packages/pisi/build.py", line 531, in build_packages
    self.gen_files_xml(package)
  File "/usr/lib/python2.3/site-packages/pisi/build.py", line 384, in gen_files_xml
    files.write(files_xml_path)
  File "/usr/lib/python2.3/site-packages/pisi/files.py", line 100, in write
    document.appendChild(x.elt(self.dom))
  File "/usr/lib/python2.3/site-packages/pisi/files.py", line 62, in elt
    sizeElt.appendChild(dom.createTextNode(size))
  File "/usr/lib/python2.3/xml/dom/minidom.py", line 1612, in createTextNode
    raise TypeError, "node contents must be a string"

pisi/files.py

@@ -55,7 +55,10 @@ class FileInfo:
         typeElt.appendChild(dom.createTextNode(self.type))
         elt.appendChild(typeElt)
         sizeElt = dom.createElement("Size")
-        sizeElt.appendChild(dom.createTextNode(self.size))
+        size = "0"
+        if self.size:
+            size = self.size
+        sizeElt.appendChild(dom.createTextNode(size))
         elt.appendChild(sizeElt)
         if self.hash:
             hashElt = dom.createElement("SHA1Sum")