Don't fail when default value for FQDN cannot be resolved.

When obtaining the hostname, the implementation of org.jivesoftware.openfire.spi.XMPPServerInfoImpl#getHostname will return a default value if another value was not explicitly defined. It is to be expected that often, an explicit value is defined (as this is done during setup). Most often, the calculated default value is unused.

At times, the default value cannot be calculated. This causes an exception to be thrown, even when the default value is not used at all.

The code should not calculate the default value, unless it is needed. Also, this calculation should have a safe fallback.