bypass dimension check if dimensions are undefined
In https://issues.cern.ch/browse/INCA-7192 an extra characteristic was introduced in ValueDescriptor to indicate that dimensions are not known at the design level. In this case, row/column count in the descriptor default to 1.
When checking dimensions, pyJapc should honor this flag and bypass the check - otherwise it always asks for a dimension of 1.
CC @ahuschau as he saw it in his WS application.