Allow zero-length strings in x-correlator header

[bigludo7]

Problem description
Allow zero-length strings in x-correlator header
As defined here camaraproject/Commonalities#387

This issue is relevant for:

• connected-network-type.yaml
• device-reachability-status.yaml
• device-roaming-status.yaml

Expected behavior

Alternative solution

Additional context

[maxl2287]

This issue is closed with #232.
(see: dfd4562)

Closing this right now.

cc @bigludo7 @akoshunyadi