Allow NULL values in Rules actions

In the 'Send HTML e-mail' rules action, some fields are optional (eg. the cc and bcc field) - which is correct.

However, when we define a (not required) node field to these rules parameters rules stops executing the action when the input of the field is empty. The warning message of rules is:

"The variable or parameter cc is empty."

Adding the property 'allow null' to the parameters solves this issue.