Filter out null as well when writing XML

[huntharo]

• I ran into an actual usage pattern where there was a null (0x00) present in a string passed to sitemap.js
• The null was written into the XML output file with no issues
• xmllint --format on the resulting file would fail with output like the following:

some.xml:1317: parser error : Char 0x0 out of allowed range
the quick brown fox jumped over the lazy dog.
                                                                              ^
some.xml:1317: parser error : Premature end of data in tag title line 1317
the quick brown fox jumped over the lazy dog.

• The filter for invalid characters being written to XML handled the range \u0001-\u0008 and others
• It looks like control characters were allowed in XML 1.1 but that NULL has never been allowed, at least according to this answer:
  • https://stackoverflow.com/a/28152666