Enhance treatment of null when the semanticSchema is not registered

[tfinez]

Class: EntityBuilderImpl Function: createEntity Description: When the getMappedModelTypes is null, there's no control over the result and following functions are executed, of course that finally there's a NullPointer exception but the result must be controlled before.
Branch: master

final Class<? extends AbstractEntityModel> entityClass = (Class<? extends AbstractEntityModel>) viewModelRegistry.getMappedModelTypes(semanticSchema.getFullyQualifiedNames());

    return createEntity(component, localization, entityClass, semanticSchema);

[azarakovskiy]

Although this means that your model is nor properly registered, this should be different exception than NPE. So basically we should add here some explicit error handling when entity class is not found instead of failing on NPE later. Good point.

[rpannekoek]

TSI-1938 (internal issue ID for tracking purposes)