kolibri: CoreTable: Do not render table data when data are loading

Follow-up to https://github.com/learningequality/kolibri/pull/10944#discussion_r1265694925

Observed behavior

In CoreTable.vue we show the circular loader when data are loading (that is when dataLoading prop truthy)

https://github.com/learningequality/kolibri/blob/0e408ee10250ef3f142352de90839328db01db75/kolibri/core/assets/src/views/CoreTable.vue#L96-L97

but there is no logic for hiding data in the table body when it is loading. That’d be fine under the assumption that whenever we’re loading data, we can be certain that there is no data yet. Generally, this might not always be true in reactive environments and when using cached data.

Expected behavior

We’d like to make CoreTable more robust by hiding table data when dataLoading prop is truthy.

User-facing consequences

Displaying two states at a time, e.g. cached data together with a loader, would be confusing.

Acceptance criteria

• There is a condition that hides data in the table body if dataLoading prop is truthy
• We can rely on exactly one state (of three - data loading, loaded but no data, loaded with data) being displayed at a time
• All places where CoreTable is used are tested for regressions