This feature request is for all
### Description [**REQUIRED**]
Currently, if the reading value is nil, it cause the system panic:
https://github.com/edgexfoundry/go-mod-core-contracts/blob/239fa787ee51d0f63f31803a319e46c292c0a290/dtos/reading.go#L255
### Describe the solution you'd like
We should use *string as the value type
🚀 Feature Request
Relevant Package [REQUIRED]
This feature request is for all ### Description [**REQUIRED**] Currently, if the reading value is nil, it cause the system panic: https://github.com/edgexfoundry/go-mod-core-contracts/blob/239fa787ee51d0f63f31803a319e46c292c0a290/dtos/reading.go#L255 ### Describe the solution you'd like We should use *string as the value type