Handle empty command lines properly.

Handle empty command lines properly.

Both para_client and para_audioc create an argument vector from the
given command line using create_argv(). If the command line contains
only whitespace characters, this vector has length zero, and argv[0]
is NULL.

We missed to check for this at at least three places in audioc.c,
client.c and audiod_command.c, which resulted in crashes due to NULL
pointer dereferences or failed assertions.

These bugs can easily be triggered by starting para_client or
para_audioc in interactive mode, entering a single space character
and hitting return.

This patch adds the missing checks to prevent the crashes.