Allow zero-length strings in x-correlator header to avoid unnecessary breaking of API clients

[hdamker]

As discussed yesterday within the TSC I see a challenge with the introduction of the pattern in #373.

API Clients which have added the the x-correlator header (as it is listed in the API definitions) but intentionally or unintentionally left it empty or set it to an empty string ("") might break due to this change:

• before the introduction of the pattern these requests were valid and could be treated as if the header was not present (see e.g. comment in the discussion: x-correlator with no value (empty / null) #366 (reply in thread))
• with the introduction of the pattern these requests will (most?) probably rejected, at least the ones who have set it to an empty string, as the introduced restricting pattern requests a minimum length of 1

My view is that API Provider should be liberal and accept NULL (no value) or empty strings within the x-correlator and treat them as if the header was not present. Within the x-correlator pattern we need to accept also strings with zero-lengths for that.

Originally posted by @hdamker in #366 (comment)