suite_state: don't return null db results

[hjoliver]

cylc suite-state fails if querying a task output message from a task proxy that exists (or existed before suite shutdown) but which has not generated its message output yet. E.g.:

[cylc]
    cycle point format = %Y
[scheduling]
    initial cycle point = 2010
    final cycle point = 2012
    [[dependencies]]
        [[[P1Y]]]
          graph = "foo:x => bar"
[runtime]
    [[foo]]
        script = cylc message "the quick brown fox"
        [[[outputs]]]
            x = "the quick brown fox"
    [[bar]]
        script = true

Run this suite to completion. It shuts down with waiting task proxies recorded in the final+1 cycle (2013):

$ cylc suite-state baz
foo, 2010, succeeded
bar, 2010, succeeded
foo, 2011, succeeded
foo, 2012, succeeded
bar, 2011, succeeded
foo, 2013, waiting  # <---
bar, 2012, succeeded
bar, 2013, waiting  # <---

Querying foo's output message is fine for succeeded instances, and non-existent instances:

$ cylc suite-state -t foo -p 2012 -m 'the quick brown fox' --max-polls=1 baz
checking for 'output: the quick brown fox': satisfied

$ cylc suite-state -t foo -p 2014 -m 'the quick brown fox' --max-polls=1 baz
checking for 'output: the quick brown fox'
ERROR: condition not satisfied

But not for the instance that exists but has not recorded an output yet:

cylc suite-state -t foo -p 2013 -m 'the quick brown fox' --max-polls=1 baz
checking for 'output: the quick brown fox''NoneType' object has no attribute 'splitlines'

This seems to be because the DB task_outputs table has a column for the messages of all existing task proxies that register outputs, unpopulated until the message is received, until which time a query returns (None, ) rather than None.

This change fixes it.

[hjoliver]

(there might be a better way to do this?)

[hjoliver]

(I'll make a test out of the example...) test added

[matthewrmshin]

No issue. The approach is good.