imp(core): Modify the entry value of the default setting to undefined…

[0x0a0d]

Limitation in the mergeSchemaSettings function (service.js#L668), where the default moleculer-db setting's entry value was set to null, preventing it from being overridden by other mixins.

By changing the default entry value to undefined, it now aligns with Lodash’s defaultsDeep method, which prioritizes values that are not undefined

[icebob]

You can overwrite these values as well, just you should reorder the mixins

[0x0a0d]

I think it is better to change null to undefined. You can bump the minor version