In §5.2.2.6 states that the eb:ConversationId element is required and must occur in the ebMS message header. In §5.2.2.10 where the element is further described it is stated that the value is "a string identifying the set of related messages that make up a conversation between Parties." and "The actual semantics of this value is beyond the scope of this specification.". There is no explicit statement on whether an empty string is allowed.
The XML schema defines the type of this element as xsd:token without any further restrictions, so an empty string would be valid. It is however unclear whether this is intentional as also the eb:Action element has the same type although the Action shall not be an empty string.
Because an empty string doesn't really qualify as an identifier it seems logical to assume that an empty value is not allowed here.
Additionally §4.3 defines also a default value of '1' for this element if no value is specified also suggesting that an empty value is not allowed.