Allow three-way merge when last applied configuration is missing

[michaelawyu]

At this moment, work controller will refuse to process objects if no last applied configuration is present (code). If the last applied configuration annotation is missing from objects for any reason (e.g. user tampering), the system will be trapped in an error loop.

During on-duty session, we have identified two clusters so far which are suffering from this pattern; since Kubernetes three-way merge algorithm itself does accept objects w/o last applied configuration (code), we could solve this pattern by dropping the requirement from our end as well.

See the discussion in on-duty channel for impact clusters.

[Arvindthiru]

#353