Be more consistent/liberal about stanza accessors.

Return the current value whenever the stanza method is called
without arguments, including when the stanza has no value.  Do
not attempt to construct an instance with no arguments (which
typically throws an exception).

Incidental comment re: depends_on

Refs #7880
Merge pull request #7935 from rolandwalker/stanza_accessors

Be more consistent/liberal about stanza accessors.