Werk #14993: reports: Fix possible KeyError on update from 2.0 to 2.1

If you had reports configured in 2.0 with datasource "customer" and a customer defined in "Context / Search Filters", the update to 2.1 showed the error "KeyError: 'customer'".

Since the UI was unusable after that error, a possible workaroud was to search for affected reports in ~/var/check_mk/web, e.g. with:

find . -name 'user_reports*' -exec grep -l "'context': {'customer'" {} \;

If you changed "'context': {'customer':'mycustomer'}" in the found files to "'context': {}" and deleted the cache file ~/tmp/check_mk/visuals_cache/cached_reports, the UI was usable again und you could configure the customer in the report again.

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