Trim null characters in Value data format (#2049)

* Trim null characters in Value data format

Some producers (such as the paho embedded c mqtt client) add a null
character "\x00" to the end of a message.  The Value parser would fail on
any message from such a producer.

* Trim whitespace and null in all Value data formats

* No unnecessary reassignments in Value data format parser

* Update change log for Value data format fix