"Send a custom email" workflow script configuration deserialisation incorrectly assumes that FIELD_EMAIL_FORMAT will be either HTML or TEXT

It seems that it can also be an empty string as shown in this redacted snippet of workflow xml submitted in a support ticket by a customer:

<function type="class">
  ...
  <arg name="FIELD_EMAIL_FORMAT"></arg>
  ...
</function>

Trying to deserialise config like this leads to:

Script failure, check logs: Cannot deserialize value of type `com.onresolve.scriptrunner.canned.jira.workflow.postfunctions.mail.EmailFormat` from String "": not one of the values accepted for Enum class: [HTML, TEXT] at [Source: UNKNOWN; line: -1, column: -1] (through reference chain: com.onresolve.scriptrunner.canned.jira.workflow.postfunctions.SendCustomEmailCommand["FIELD_EMAIL_FORMAT"])
Cannot deserialize value of type `com.onresolve.scriptrunner.canned.jira.workflow.postfunctions.mail.EmailFormat` from String "": not one of the values accepted for Enum class: [HTML, TEXT]