Fixes #2250 - Don't show loader after the collection has been loaded.

[lindapaiste]

Fixes #2250

Changes:

• Change the defaultProps for collection from an incomplete/fake object to null. This way it's easy to know when the collection has been loaded.
• <Loader/> component won't be displayed after this.props.collection has been set, regardless of the value of this.props.loading.
• hasCollection only looks to see if it has a valid collection prop and does not consider this.props.loading. Consequently the collection details will stay visible while the popup content is loading.
• The empty table will be displayed whenever there is a collection but it has no items. Loading is not considered. The change in the defaultProps makes this check possible.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

[raclim]

Thanks for catching and working on this!