[core-amqp] validate() will string-ize 'undefined'

[richardpark-msft]

In #8074 we discovered that core-amqp's validate() method will helpfully do a string-izing of the entity path field in the connection options.

Unfortunately, if the field is undefined it actually stringizes it instead, leaving you with entityPath: 'undefined'.

We should fix this and just make sure nobody is depending on it working that way in our code (EH and SB).

(NOTE: SB should be fine but you can remove the workaround I added after you fix this)