[lucene] instantiation issue with the LuceneOptimizerManager

[pmauduit]

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).

[PascalLike]

If the error message "Parameters saved but cannot restart Lucene Index Optimizer: null" is thrown from here https://github.com/geonetwork/core-geonetwork/blob/cc04086527c8b47de54cfa332aab94240e9788ba/services/src/main/java/org/fao/geonet/api/site/SiteApi.java#L137 it calls this piece of code, where these attributes are initialized: https://github.com/geonetwork/core-geonetwork/blob/cc04086527c8b47de54cfa332aab94240e9788ba/core/src/main/java/org/fao/geonet/kernel/search/LuceneOptimizerManager.java#L110-L111

[pmauduit]

it calls this piece of code, where these attributes are initialized:

This code is not reached, because of the NPE thrown at line 106 (_beginAt being null)

[PascalLike]

Right! So better make this code more robust.