mod_report_forward: Fix traceback on error message passed to datetime function If archive:get() returns an error, e.g. because it can't find a message with the given ID, it may return e.g. nil, "item-not-found" or similar where the error condition ends up in reported_message_time, which gets passed to util.datetime, which thus ends up doing math to a string or error object.

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labels:
- 'Stage-Beta'
summary: 'XEP-0377: Spam Reporting'
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This module is a very basic implementation of [XEP-0377: Spam Reporting].

When someone reports spam or abuse, a line about this is logged and an
event is fired so that other modules can act on the report.