A recent change[1] to 20x11-common_process-args causes
"/etc/X11/Xsession true" to pop up an xmessage error, followed by
running the default session.

The default xpra configuration contains such an invocation in order to
run /etc/X11/Xsession.d/* without  actually running ~/.xsession.

The reason is that "command -v true" returns "true", which fails the
test for both -e and -x.

Given that "command -v program" already checks if program exists and is
executable, then the test may be simplified to just that invocation.

[1] https://salsa.debian.org/xorg-team/xorg/-/commit/3d6bd60c17ff052fc3f35eb0f17553fcf3d0b23f