feat(ui): disable export when collection is empty

[kobenguyent]

resolves #230

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[flawiddsouza]

This seems like extra processing when not needed. You can use this.$store.state.collection.length instead. It will have the same data. As this is just for disabling the export button.

[flawiddsouza]

Make it a computed property instead of using a data property - this will allow collectionLength to be dynamic. No need to use beforeMount.