Implement #67331: Have parse_ini_file add empty entries

Implement #67331: Have parse_ini_file add empty entries

Some INI processors allow to specify empty values by just giving the
key without the equals sign, for instance MySQL and Python.  It appears
to be sensible to add this possibility to our INI parser, so that it
can be used for such INI files as well.  We choose NULL as the value of
empty values.

This syntactical enhancement is a (minor) BC break, though, as can be
seen by the necessary change to bug49692.ini.  The “comment” formerly
has been simply ignored, but now it would be parsed as key with an
empty value.

This PR is based on Adam's former patch.