add unit tests for reading empty values with the client
add unit tests for reading empty values with the client

open62541/open62541,jpfr/open62541,open62541/open62541,open62541/open62541,JGrothoff/open62541,jpfr/open62541,JGrothoff/open62541,JGrothoff/open62541,jpfr/open62541,open62541/open62541,StalderT/open62541,jpfr/open62541,StalderT/open62541,JGrothoff/open62541,StalderT/open62541,StalderT/open62541