rcxml: discourage empty strings in rc.xml configuration

rcxml: discourage empty strings in rc.xml configuration

Background:
I rewrote the config parser in 9462457..2f414a4, but it broke certain
configurations by changing how empty strings are handled: they were mostly
just ignored before my parser rewrite, but after that, they are
interpreted as just empty strings (output="" is considered as 'output named ""').

Though that was unintentional, I believe ignoring empty strings was not a
good idea in the first place, as we already allow empty strings for
certain configurations (e.g. `<desktop prefix="">`), which makes the
parser's behavior inconsistent.

Change:
So let's clarify that we intend to read empty strings as empty strings.
As a preparation, this commit adds warnings for empty strings we are
currently ignoring, so that users can be informed that we intend to just
read empty strings (e.g. `<theme name="">`) as empty strings in the future.
I removed existing empty strings in `rc.xml.all` to avoid warnings when
reading it.