[datadog_security_monitoring_suppression] Fix error handling when updating suppression payload

[clementgbcn]

We discovered that providing an empty expiration date/badly formatted expiration date led to a kernel panic:
invalid memory address or nil pointer dereference.

It was due to a missing return after getting an error.

After adding this change, we did a local test to check that the error was properly handled.

[pietrodll]

Thanks 🙌

[clementgbcn]

/merge

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