command: handle empty buffer argument correctly

command: handle empty buffer argument correctly

virBufferContentAndReset (intentionally) returns NULL for a buffer
with no content, but it is feasible to invoke a command with an
explicit empty string.

* src/util/command.c (virCommandAddEnvBuffer): Reject empty string.
(virCommandAddArgBuffer): Allow explicit empty argument.
* tests/commandtest.c (test9): Test it.
* tests/commanddata/test9.log: Adjust.