Support blank default values for auto-intent macro variables without causing decode errors

NetBrain currently requires a default value to be specified for a macro variable used in an auto-intent; leaving the default blank causes the associated command to fail decoding, since the command becomes invalid without a value in place. This enhancement would allow a macro variable's default value to remain empty while preserving valid decode logic, so the command does not error out when no default is supplied. Supporting this would let customers force users to manually enter required input on every execution, improving accuracy and control over auto-intent runs without sacrificing decode reliability.