If you refer to the UPA Dev Guide, in section 11.2 Primitive Types, it states:
When contained in an RsslFieldEntry or RsslElementEntry, base primitive types can also represent a blank value. A blank value indicates that no value is currently present and any previously stored or displayed primitive value should be cleared. When decoding any base primitive value, the interface function (See Table 69) returns RSSL_RET_BLANK_DATA
I am not saying that data for every exchange / asset class will necessarily get a blank a field during exchange auction period / trading suspension etc. However, if the feed does want to indicate if a particular currently has no value or it's existing value needs to be cleared, then a blank field as described above should be present in the payload.
For the actual behaviour for a particular exchange / asset class - I would recommend you create a Content type query with our helpdesk so that a Content specialist can advise you accordingly.
If you are using OMM Data / RSSL interface to consume your data then an empty field can be checked using the isBlank() or similar function on the field's data container.
If however, you are using legacy MarketFeed data format - then please note that MarketFeed can use "+0" to represent a blank price field for example.