AMS sending silently ignores empty messages

[tofu-rocketry]

Following #247, I noticed that we're silently ignoring empty messages.

ssm/ssm/ssm2.py

Line 493 in 0ec0b31

argo_id = self._send_msg_ams(text, msgid)

_send_msg_ams will return None for an empty message and the code here will log as if the message was sent:

ssm/ssm/ssm2.py

Line 495 in 0ec0b31

log_string = "Sent %s, Argo ID: %s" % (msgid, argo_id)

We should wrap that in an if...else and log a different message if the message wasn't sent, perhaps a warning.

[RedProkofiev]

_send_msg_ams in its current implementation will not return a None under effectively any circumstance. A newline from a touch'd, empty file or some random 0's or more importantly the empty string will all render as "something", because only None can be None. We could try to handle this by handling for the empty string, making it return a None and not bother sending if the file itself has no characters bar newlines, but I'm reluctant to filter any more. After that, the if...else handling around 493 should work fine. Do you think that's worth implementing?