Allow default initializer when all fields are initialized

[KunJeong]

As discussed in the last LDWG meeting regarding p4c#5265, we would like to mention that it is allowed to use ... when all fields have been initialized. This PR adds the bolded line to 8.27 Initializing with default values of the spec:

A left-value can be initialized automatically with a default value of the suitable type using the syntax ... (see sec-default-values). A value of type struct, header, or tuple can also be initialized using a mix of explicit values and default values by using the notation ... in a tuple expression initializer; in this case all fields not explicitly initialized are initialized with default values. It is permissible to use ... when all fields have been explicitly initialized, in which case it has no effect. When initializing a struct, header, and tuple with a value containing partially default values using the ... notation the three dots must appear last in the initializer.