Fixed AWSSDRegistry nil labels map

[voro015]

Description
Running external-dns for node source with aws-sd registry will crash on initial loop due to labels being nil map.
IDK what would be more suitable fix here:

• to add a simple check to init a map if Endpoint Labels are nil in aws-sd registry
• to init Labels map with NewLabels() when creating Endpoints for node source.

Fixes #2094

Checklist

• Unit tests updated
• End user documentation updated

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[Raffo]

@voro015 Please sign the CLA.

[voro015]

@Raffo sorry, it took some time, now should be signed.

[njuettner]

@voro015 super cool thank you 🚀 . it seems this was falling through the cracks because of missing unit tests. do you mind adding a test for that which covers this issue?

[voro015]

@njuettner yeah, I added simple test, please have a look.
Also, should I keep both "fix" for a nil labels? Like that one within aws-sd registry fixing it only issue "symptoms", same time one within node source fixing its root.

[njuettner]

Should be fine for now, I'll merge it. Thanks again.

[njuettner]

/lgtm

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: njuettner, voro015

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