Dashboard Migration: Fix v38 table panel overrides migration when defaults.custom is missing
The v38 migration was incorrectly skipping field config overrides processing
when fieldConfig.defaults.custom didn't exist. This caused custom.displayMode
properties in overrides to not be migrated to the new custom.cellOptions format.
The fix ensures migrateOverrides is always called for table panels, regardless
of whether defaults.custom exists.
Added comprehensive unit test covering this edge case to prevent regression.
Issue discovered through comprehensive migration testing infrastructure.
Dominik Prokop
26d36ec7ff