fix(GlLegend): Show dash on multiple empty cases

What does this MR do?

Fixes first part of gitlab#228542 (closed).

Show en-dash when the series data array is not empty, but contains only NaN values.

In this case, we would see a dash (-) as in other empty cases.

Motivation

Turns out that certain Prometheus queries can give us entire series full of NaN values.

Which result in hundreds of errors:

Does this MR meet the acceptance criteria?

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