Potential bugs: NULL strings from svn_tristate_to_word()

[Julian Foad <ju...@wandisco.com>]

In svn/log-cmd.c:log_entry_receiver_xml(), svn_xml_make_open_tag() is
called with attribute values from tristate_to_word that may be NULL, and
this is not documented as acceptable.

In mod_dav_svn/reports/log.c:log_receiver(), dav_svn__brigade_printf()
is called with "%s" arguments from tristate_to_word that may be NULL,
and this is not documented as acceptable.

What is the intended behaviour?

In the first case, it looks like the implementation will omit any XML
attribute whose value is given as NULL, and it looks to me like that's
the intended behaviour, so I suggest we document it.

In the second case, I'm not sure what's intended or what happens.

- Julian