When async-shell-command-display-buffer is non-nil, the output buffer is
displayed immediately.  Otherwise, the buffer is displayed by the
(advised) process filter, which should only be invoked for
output-producing shell commands[1].  For example, the Unix null command
M-& : RET does not invoke the process filter.

Without my patch, the process filter advice only displays the output
buffer if it is already empty, i.e. if no previous shell-command
invocation has written output to the process buffer.  This guard seems
to contradict the purpose of a non-nil shell-command-dont-erase-buffer
setting.

So, my patch continues to allow the immediate display of the output
buffer, irrespective of command output, when
async-shell-command-display-buffer is non-nil, as well as the deferred
and repeated display of the output buffer, irrespective of
shell-command-dont-erase-buffer and subject to command output, when
async-shell-command-display-buffer is nil.

[1] This assumption is based on observation, not expertise in process
    filters; I cannot guarantee the absence of obscure edge cases.

-- 
Basil