Add placeholder text instead of an empty string when the histogram for a query is empty

When the histogram for a query is empty we currently render an empty string, which makes it look like it failed to render. We should instead render some text saying that there is an empty histogram, or simply render an empty histogram.

This is more important following !63 (merged) because we'll ignore rails internal queries in histograms, so more histograms will be empty.

The following discussion from !63 (merged) should be addressed:

@dfrazao-gitlab started a discussion: (+1 comment)

Why are we removing this part?

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