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.

tests/commanddata/test9.log

@@ -2,8 +2,10 @@ ARG:-version
 ARG:-log=bar.log
 ARG:arg1
 ARG:arg2
-ARG:arg3
+ARG:
 ARG:arg4
+ARG:arg5
+ARG:arg6
 ENV:DISPLAY=:0.0
 ENV:HOME=/home/test
 ENV:HOSTNAME=test