Describe the bug
Publishing an empty message to config_snapshot, config_update, and log_upload command metadata topics invokes an error message.

EOF while parsing a value at line 1 column 0

Publishing an empty message to these topics are expected usecases since we need to publish empty retained messages to cancel the capabilities.

To Reproduce
Publish these messages below. (change the child1 part to your device external ID)

tedge mqtt pub -r te/device/child1///cmd/config_snapshot ''
tedge mqtt pub -r te/device/child1///cmd/config_update ''
tedge mqtt pub -r te/device/child1///cmd/log_upload ''

Then you always get the same error messages on te/errors topic.

[te/device/child1///cmd/config_snapshot] 
[te/errors] EOF while parsing a value at line 1 column 0
[te/device/child1///cmd/config_update] 
[te/errors] EOF while parsing a value at line 1 column 0
[te/device/child1///cmd/log_upload] 
[te/errors] EOF while parsing a value at line 1 column 0

Expected behavior

No error messages.

Screenshots

Environment (please complete the following information):

• OS [incl. version]
• Hardware [incl. revision]
• System-Architecture [e.g. result of "uname -a"]
• thin-edge.io version [1.0.1]

Additional context