Fix JENKINS-13242 - global settings always overriding husdonUrl

[hajush]

Only overwrite the email-ext setting for Jenkins URL when the checkbox is marked. This doesn't matter if the email-ext for Jenkins URL is empty, but if it is non-null, without these changes - the non-null settings get used for all email content generation scripts as the ${rooturl}, since it fetches it directly from this class - ExtendedEmailPublisherDescriptor - without checking against the overrride setting.

[slide]

Does this really do what you are expecting?

[hajush]

I've tested it and it performs what I think was the intent of the original design. If on startup the value is null or empty for the email extension override for jenkins url - it gets filled with the current jenkins url. So the empty value there is not preserved.

Before this change, if the email extension jenkins url is changed to something different than the hudsonUrl, but then the override is turned off - the overridden value continues to be used. Clearly broken. And not that easy to figure out how to fix for a user. My small change fixes that.

What it does now is if you turn off "override", then any value that was entered into the email extension jenkins url field will be overwritten if you turn override off and then save. It's not the most intuitive - but it fits with the original design that the field gets filled with the current hudsonUrl setting and is certainly a lot less code and design change.