EmailActionExecutor does not use a default from-address as documented

Oozie's docs have:

=oozie.email.from.address= - The from address to be used for mailing all emails (oozie@localhost by default).

(From http://incubator.apache.org/oozie/docs/3.1.3/docs/DG_EmailActionExtension.html)

However, the code does not seem to use a default when loading the prop from the Oozie conf. Thus if someone does not specify it, it causes an NPE when it tries to from a fromAddress object later.

Same applies to:

oozie.email.smtp.host - The host where the email action may find the SMTP server (localhost by default).