Fixed NPE in Cleaner Scheduler after migrating to OSGi 6

[malaskowski]

Description

After #326 company and project in the cleaner configuration didn't provide non-null default value and caused:

aet_karaf.1.nkvm7vv3tis4@default    | 12:26:32.716 ERROR [fileinstall-/opt/karaf/deploy] Fatal error while activating cleaner scheduler
aet_karaf.1.nkvm7vv3tis4@default    | java.lang.NullPointerException: null value in entry: companyFilter=null
aet_karaf.1.nkvm7vv3tis4@default    |   at com.google.common.collect.CollectPreconditions.checkEntryNotNull(CollectPreconditions.java:32)
aet_karaf.1.nkvm7vv3tis4@default    |   at com.google.common.collect.ImmutableMap.entryOf(ImmutableMap.java:176)
aet_karaf.1.nkvm7vv3tis4@default    |   at com.google.common.collect.ImmutableMap$Builder.put(ImmutableMap.java:285)
aet_karaf.1.nkvm7vv3tis4@default    |   at com.cognifide.aet.cleaner.CleanerScheduler.registerCleaningJob(CleanerScheduler.java:123)

Before empty string values were provided.

Motivation and Context

Fixed NPE.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

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