[lucene] instantiation issue with the LuceneOptimizerManager

Considering the following code: https://github.com/geonetwork/core-geonetwork/blob/develop/core/src/main/java/org/fao/geonet/kernel/search/LuceneOptimizerManager.java#L70-L74

If the first state of the optimizer is `enabled` set to `false`, the `_beginAt` member variable would never be initialized, causing any change in the admin UI (activating / deactivating the index optimizer) to fail with the following message:

```
Parameters saved but cannot restart Lucene Index Optimizer: **null**
```

Following the generated stacktrace lead me here:
```
https://github.com/geonetwork/core-geonetwork/blob/develop/core/src/main/java/org/fao/geonet/kernel/search/LuceneOptimizerManager.java?utf8=%E2%9C%93#L106
```

I guess the correct behaviour would be, regardless of the `enabled` value, to set the `_beginAt` variable in the constructor (even if the bean is actually deactivated).