fix: allow coderd to start with an empty DERP map when built-in DERP is disabled (#24544)

Allow coderd to start with an empty base DERP map when built-in DERP
is disabled and no static DERP map is configured, so DERP can come from
workspace proxies after startup.

Also add a DERP healthcheck warning when no DERP servers are currently
available at runtime.

Related to: https://linear.app/codercom/issue/PLAT-43/bug-coderd-unable-to-be-started-if-built-in-derp-server-disabled-and
Related to: https://github.com/coder/coder/issues/22324

coderd/apidoc/docs.go

@@ -23736,6 +23736,7 @@ const docTemplate = `{
                 "EACS04",
                 "EDERP01",
                 "EDERP02",
+                "EDERP03",
                 "EPD01",
                 "EPD02",
                 "EPD03"
@@ -23756,6 +23757,7 @@ const docTemplate = `{
                 "CodeAccessURLNotOK",
                 "CodeDERPNodeUsesWebsocket",
                 "CodeDERPOneNodeUnhealthy",
+                "CodeDERPNoNodes",
                 "CodeProvisionerDaemonsNoProvisionerDaemons",
                 "CodeProvisionerDaemonVersionMismatch",
                 "CodeProvisionerDaemonAPIMajorVersionDeprecated"