The connectivity-overrides section still documented
`hyperhive.forge.url = "http://localhost:3000"` as the default and said
the empty string was a valid value. Both stopped being true when the
option became `nullOr str` with `default = null`.
Also says what `null` means and why, since "the default is null" alone
reads like an oversight rather than the point: a loopback default is
only correct when the forge shares the agent's netns, and the units that
would consume the URL are simply not generated without one.
Refs #2860
f3569160d3