Assert preconditions in ResourceDatabasePopulator

Prior to this commit ResourceDatabasePopulator did not assert any
preconditions for constructor and method arguments. Consequently,
errors would not be encountered until the populator was executed.

This commit addresses this issue by ensuring that preconditions for
constructor and method arguments are asserted immediately, throwing
IllegalArgumentExceptions where appropriate.

Issue: SPR-11690
Add script after setting scripts in RsrcDbPopultr

Prior to this commit it was impossible to add additional scripts to a
ResourceDatabasePopulator after setScripts() had been invoked.

This commit fixes this by ensuring that the internal scripts list
continues to be modifiable when setScripts() is invoked.

Issue: SPR-11691