Yaml parser should allow empty elements to be expressed as 'null' or '{}'

According to the Yaml "spec" null objects can be specified as:

 null | Null | NULL | ~	 tag:yaml.org,2002:null
 /* Empty */	 tag:yaml.org,2002:null

Currently only ~ is supported.

This causes issues if the Infinispan yaml is transformed by a 3rd party resource, e.g. Helm Charts or K8s, and ~ is replaced with any of the other values.